BIBLIOGRAPHIC REFERENCE

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2 BIBLIOGRAPHIC REFERENCE Morgenstern, U., van der Raaij, R., Baalousha, H Groundwater flow pattern in the Ruataniwha Plains as derived from the isotope and chemistry signature of the water, GNS Science Report 212/23 5p. U. Morgenstern, GNS Science, PO Box 3368, Lower Hutt R. van der Raaij, GNS Science, PO Box 3368, Lower Hutt H. Baalousha, Hawkes Bay Regional Council, 159 Dalton Street, Napier 411 Institute of Geological and Nuclear Sciences Limited, 212 ISSN ISBN i

3 CONTENTS ABSTRACT... III KEYWORDS... IV 1. INTRODUCTION PREVIOUS INVESTIGATIONS HYDROGEOLOGY AND CATCHMENT CHARACTERISTICS SAMPLE LOCATION AND BORE DATA HYDROCHEMISTRY National context Variations in hydrochemistry within the study area Hydrochemistry variation and trends over time STABLE ISOTOPES AND RADON AS INDICATORS OF RECHARGE SOURCE RECHARGE TEMPERATURES AND EXCESS AIR CONCENTRATIONS AS INDICATORS OF RECHARGE SOURCE GROUNDWATER DATING Methodology of groundwater dating Interpretation of tritium time series and gas tracer data DISCUSSION RECHARGE SOURCE AND FLOW DYNAMICS CONCLUSION ACKNOWLEDGEMENTS REFERENCES FIGURES Figure 1 Ruataniwha Basin geology map showing the location of sampling sites... 3 Figure 2 West-east geological section of the Ruataniwha Basin... 5 Figure 3 North-south geological section of the Ruataniwha Basin... 6 Figure 4 Dendrogram produced by hierarchical cluster analysis Figure 5 Piper diagram showing the variation of major ion chemistry in the Ruataniwha samples Figure 6 Geographic distribution of sites assigned to hierarchical cluster analysis-defined clusters Figure 7 Water level and hydrochemistry trends of well Figure 8 Water level and hydrochemistry trends of well Figure 9 Water level and hydrochemistry trends of well Figure 1 Water level and hydrochemistry trends of well Figure 11 Plot of δ 18 O against δ 2 H for Ruataniwha sites Figure 12 Plot of δ 18 O against chloride Figure 13 Plot of nitrogen versus argon concentrations, normalised to sea level Figure 14 Plot of excess air against chloride concentrations Figure 16 Groundwater MRT s in years, together with bore depth Figure 17 Depth versus MRT for the Ruataniwha Basin and Linkwater-Malborough Figure 18 Redox sensitive parameters DO, Fe, Mn, CH 4, NH 3, and B versus MRT in years Figure 19 ph, HCO 3, SiO2, and F versus MRT in years Figure 2 NO 3 and PO 4 versus MRT Figure 21 Spatial distribution of the indication of recharge sources GNS Science Report 212/23 i

4 TABLES Table 1 Annual rainfall data for the Ruataniwha Basin Table 2 Well identification, location, construction details, water level, aquifer lithology and condition, and land-use information... 8 Table 3 Chemistry data for Ruataniwha Basin sites... 9 Table 4 Values for field parameters, radon, and stable isotopes Table 5 Dissolved argon and nitrogen concentrations and derived variables Table 6 Tritium, SF 6, and CFC results, together with groundwater age parameters Table 7 Indication of recharge source as derived from the various methods GNS Science Report 212/23 ii

5 ABSTRACT The Ruataniwha Basin is situated in the upper Tukituki catchment, approximately 7 km south west of Napier City. The boundaries of the Ruataniwha Basin are the foothills of the Ruahine Range in the west, Turiri Range and Raukawa Range in the east and rolling hills in the north. The Ruataniwha Plains groundwater system is a multi-layered aquifer system that has a complex hydrogeological setting, as the plains evolved in response to sea-level changes, tectonic activity, and geomorphic processes. Aquifers in the basin occur in gravel, sandstone, pumice and limestone strata within a basin structure. In this study, groundwater samples have been collected for hydrochemistry, dissolved gases, and age tracer analysis. Tracer results were interpreted in terms of groundwater recharge source and rate, groundwater age, changes in groundwater source, and the homogeneity of the aquifers. This helps with conceptual understanding of Ruataniwha Basin groundwater flow patterns, and provides data for calibration of a numerical surface-groundwater flow model. Most water samples across the Ruataniwha Basin contain old water, with a mean residence time (MRT) > 25 years. Only one well (376) has younger water and it is likely to be linked to the river via a fast flow path such as paleo river channels. The old age of most of the waters indicates that these groundwaters are not directly linked to surface water. In the south eastern part of the basin, all groundwater samples are old (>1 years), indicating slow movement of groundwater and slow recharge, consistent with the geology of the area. In the south eastern part of the basin the geologic units have low permeability. The age depth relationship is biased by upwelling groundwater and reflects the closed nature of the basin. The average vertical flow velocity indicates a recharge rate of.19 m/y. Four wells in the vicinity of the lower Waipawa River show excellent age-depth relationships, indicating absence of disturbance by groundwater upwelling. The recharge rate there of.42 m/y is substantially higher than in the other parts of the basin, indicating river recharge in addition to recharge by rain. These recharge rates represent averages (i) across the basin, and (ii) over the last hundred years, because they were deduced from water age data covering the last hundred years, and covering the whole basin. Hydrochemistry data for the Ruataniwha Basin show high variability in space and time, consistent with the complicated hydrogeological setting, which consists of multi-layered aquifer systems in different lithologic strata and pockets of gravels. Groundwaters with extreme hydrochemistry were found, including high phosphate (>1 mg/l) and ammonia (>4 mg/l) from natural sources, and extremely low silica (<.1 mg/l) in stagnant deep old groundwater. Alternating hydrochemistry over time in several deep wells indicates that pockets of water with different hydrochemistry exist in close proximity. The use of a combination of water isotopes ( 18 O and 2 H), dissolved gases (Ar and N 2 ), and hierarchical cluster analysis of the hydrochemistry parameters allowed characterisation of the recharge source for most of the groundwater samples. Only groundwaters in the vicinity of the large rivers (Waipawa and Tukituki) show river recharge source a signature, indicating gravel deposits connecting the present river bed to the deep groundwater flow system along these rivers. River-recharged groundwater is observed only in the lower reaches of these rivers, downstream from losing stretches of the rivers. Oxic groundwater is present only in GNS Science Report 212/23 iii

6 the vicinity of the Waipawa River, indicating that only this river has deposited relatively clean gravel aquifers (without organic matter that would otherwise deplete the oxygen). A very distinct river recharge signature is observed in the groundwater of well 4694, in a gaining stretch of the river, close to where the Waipawa River exits the basin. This indicates that the groundwater in this area is river water that was lost from the river further upstream and is upwelling back to the surface at the end of the basin. The springs in this area (Limestone and Drainage channel) show a mixed signature, between rain and river recharge, indicating shallow groundwater with a mix of river and rain recharge. The groundwater locations with an overall indication of river recharge are: Wells 143, 1518, 243, 378, 372, and 4694, and the Drainage channel spring. Wells with an overall indication of local rain recharge are: 1381, 1452, 1558, 1655, 1944, 2579, 314, 3426, 3852, 6719, 1942, and 115. All the groundwaters in the southern part of the basin in the vicinity of small rivers and streams show a pure rain recharge signature. This indicates that there is little connection of rivers and streams there to the deep groundwater system in this area. All samples follow similar and consistent trends of hydrochemistry versus age, indicating that, despite the complex structure of the groundwater system with localised heterogeneity at basin-wide scale, groundwater in the basin overall has a homogenous flow pattern. KEYWORDS Hydrogeology, Ruataniwha Plains, hydrochemistry, isotopes, groundwater age, tritium, CFCs, SF6, recharge source, recharge rate, groundwater flow pattern. GNS Science Report 212/23 iv

7 1. INTRODUCTION Groundwater is an important resource for the Ruataniwha Plains, Central Hawkes Bay, providing water for farm, domestic and stock supplies, and orchard, crop and pasture irrigation. Water use (groundwater and surface water) is currently low compared to inflow/outflow components (Baalousha, 21). However, there is a high potential for irrigation in the basin and demand for water is expected to increase. Previous investigations covered mainly the physical hydrogeology of the Ruataniwha Basin. More recent work has been done to develop a transient groundwater flow model (Baalousha, 21). To enable sustainable use of the groundwater, a flow model has been developed to assist the setting of allocation limits for the basin. This report presents the results of age tracers, gases, and chemistry time trends. This study is intended to improve our understanding of the dynamics of the Ruataniwha Plains groundwater system, including refinement of the conceptual groundwater flow model, identification of the recharge sources and rates, surface-groundwater interaction, and age data for direct calibration and validation of the results of the numerical groundwater flow model. 2. PREVIOUS INVESTIGATIONS The potential for the Ruataniwha Plains basin structure to contain a layered aquifer system, including flowing aquifers, was first recognized by Hill (1893). Following Hill s advice, Mr Harding of Mount Vernon Station drilled a 96.3 m deep well about 1.5 km northeast of Ongaonga Township. The driller was Mr J. Gilberd of Napier. At a depth of 78.9 m the well encountered shingle with the water level 5 m above ground level (Ongley, 1937). The flowing aquifer continued to 96.3 m. For the next 9 years, wells were dug and drilled on the Ruataniwha Plains to supply domestic and stock water. In the 198s, conversion of dry land farming to orchards and dairy farms on the Ruataniwha Plains resulted in wells being drilled for irrigation water supply. Concern regarding the capacity of the aquifers to sustain high yields without depleting the interconnected surface water sources, especially the Waipawa and Tukituki Rivers, and the impact of intensified water and land use on surface water and groundwater quality promoted investigations by the Hawke s Bay Catchment Board (McGuiness, 1984; Ludecke, 1988). These investigations initiated groundwater data acquisition programmes. In the 199s, the Hawkes Bay Regional Council (HBRC) carried out a regional Ruataniwha Plains groundwater review study (Dravid, 1993), which indicated there could be at least four aquifer groups (including deep Tertiary strata aquifers) underlying the Ruataniwha Plains. The recommendations of this review were incorporated in a 2-year joint HBRC and Institute of Geological and Nuclear Sciences Ruataniwha Plains groundwater study in The study incorporated geology and groundwater isotope analyses into the interpretation of previous studies and the groundwater database. This joint study was not completed and some of the results were collated in Dravid et al. (1997) and Brown (1998). More results were included in Pattle Delamore Partners (1999). These later studies also presented results of groundwater chemistry and isotope (tritium and oxygen-18) monitoring, which shows distinctive groundwater types related to groundwater age and aquifer depth. GNS Science Report 212/23 1

8 Meilhac et al. (29) summarised hydrogeological methods being used in the investigation of groundwater-surface water interactions around the Waipawa River in the Ruataniwha Plains. That study included development of a detailed potentiometric surface, application of heat tracing to calculate streambed conductivity; use of surface geophysical methods to assess aquifer hydraulic properties; application of slug testing and comparison to properties determined by pump testing; and use of borehole geophysical techniques such as natural gamma logging. Undereiner et al., (29) focused on groundwater-surface water interactions around the Waipawa River. River gauging indicated losses to groundwater along the Waipawa River of around 2.2 m 3 /s, and inflows from groundwater of around the same magnitude to the Mangaonuku and Kahahakuri Streams. A 3D geological model created from lithological logs of 288 bores indicated two aquifer systems, a shallow unconfined aquifer up to 75 m thick and a deeper and thicker confined aquifer. The two aquifers may merge in the south eastern area of the Ruataniwha Plains. Evaluation of chemistry data highlighted a relationship between water from the Waipawa River and that of the Kahahakuri Stream, as well as the shallow aquifer and outflowing springs, and a relationship between water from the Mangaonuku Stream and that of the deep aquifer. A water budget developed for the area identified groundwater pumping as being only a small proportion of the total budget. Baalousha (28a, 28b, 21) developed steady and transient groundwater-surface water flow models for the Ruataniwha Basin. The finite difference-based Modflow model was used for that study (Harbaugh et al., 2). Steady-state conditions were assumed in 199, where the groundwater abstraction was negligible, and stresses on the water resources were small. The results of the steady-state model have been used as the initial head condition for the transient model. The transient model uses the stream package of Modflow (SRF1) (Fenske et al., 1996) to simulate stream routing of 12 rivers and streams in the basin. Results of the model show that aquifer-river interaction is high. In terms of volume, river gain from the aquifer is high compared to loss from the river to the aquifer, with spatial variation in the gainloss relationship. Rivers generally lose water to the aquifer upstream, and gain water downstream from it because the Ruataniwha Basin is closed and the only groundwater outflow is via seepage to rivers downstream. 3. HYDROGEOLOGY AND CATCHMENT CHARACTERISTICS The Ruataniwha Basin is relatively young in geologic terms, with an age of less than 1.5 million years. It was formed as a result of uplift of the Ruahine Ranges to the west and the Otane Anticline to the east of the basin, followed by continuous erosion and deposition of sediments by the rivers in the lower altitude areas of the basin. Extensive areas of the basin were part of the ocean up to.5 million years ago, as indicated by an in-fill of marine deposits. As a result of ongoing tectonic uplift, the basin separated from the sea, and erosion of rock and soil material from the Ruahine ranges filled the basin with sediments (Figure 1). GNS Science Report 212/23 2

9 Figure 1 Ruataniwha Basin geology map showing the location of sampling sites with their ID numbers and approximate bore depth, and rainfall stations. The basin is composed mainly of sequences of alluvial gravel with intermittent clay layers of different thicknesses deposited during alternating cycles of glacial and temperate climate during the Quaternary period. Two main gravel layers occur in the basin. The top is composed of a young gravel layer from the late Pleistocene and Holocene epoch situated over an older gravel layer, the Salisbury gravel from the early Pleistocene. The Young Gravel Formation is unconsolidated and contains clay, silt and volcanic ash of late Quaternary age (Francis, 21). It is more permeable than the underlying Salisbury gravel, which is composed of poorly consolidated gravel, ignimbrite and clay of Lower Quaternary age (Francis, 21). The thickness of gravel layers varies from a few meters in the west to about 2 m in the middle of the basin. The groundwater flow is generally from north-west to south-east, and is almost parallel to river flow. The basin is hydrogeologically closed, as there is no lateral groundwater flow into or out of the basin because the basin gravel aquifers are contained within the hard rock geology of the basin margins. The basin aquifers are recharged by rainfall and from the GNS Science Report 212/23 3

10 rivers. Groundwater discharge from the basin occurs as groundwater seepage into the Waipawa and Tukituki rivers, which exit the basin in the east, in addition to groundwater abstraction and surface water takes. Schematic geological cross sections through the middle of the basin are shown in Figure 2 (West-East) and Figure 3 (North-South). The top layer of the Young Gravel Formation is thin and does not extend over the whole basin, whereas the old gravel (Salisbury) is thick and extends further to the west. The gravel formations are underlain by thick impermeable mudstone and siltstone. The geology of the basin is heterogeneous and distinct aquifers are unknown. Pockets of gravels that cannot be identified as separate distinct aquifers are likely to have a high connectivity. The majority of shallow wells (less than 4 m deep) are generally unconfined, while the deep wells are mainly confined. The deepest gravel aquifer has been identified at a depth of 134 m, with the deepest utilised well screened from 114 to 118 m (Brown, 22). Three main rivers and streams traverse the basin from west to east: Waipawa River in the north, Tukituki River in the middle, and Makaretu Stream in the south. In addition there are a number of smaller streams in the basin. All rivers and streams merge at the eastern edge of the basin into two rivers; the Waipawa and the Tukituki, which meet some 1 km outside the basin, a few kilometres to the east of the towns of Waipawa and Waipukurau. Within the basin there is a marked interaction between groundwater and surface water. The flow patterns in rivers and streams within the basin vary according to a loss-gain relationship between aquifers and streams. To promote the long-term sustainability of the groundwater resource, an understanding of both the flow system and this loss-gain relationship is needed. GNS Science Report 212/23 4

11 Figure 2 West-east geological section of the Ruataniwha Basin (Pattle Delamore Partners, 1999). GNS Science Report 212/23 5

12 Figure 3 North-south geological section of the Ruataniwha Basin (Pattle Delamore Partners, 1999). GNS Science Report 212/23 6

13 DRAFT 212 The climate in the Ruataniwha Basin is characterised by high rainfall in winter, with annual values ranging from 8 mm in the east to more than 16 mm on the Ruahine Ranges to the west (Table 1). Daily average January temperature s range from 13. C to 24.2 C, and July temperatures range between 3.2 C and 13.8 C. Table 1 Annual rainfall data for the Ruataniwha Basin. Year Brinkswey Chesterman Rain at different stations [mm] Parkhill Punanui Tikokino Average [mm] Min Mean Max Unlike rainfall, evapotranspiration shows little variation (Thompson, 1987). The maximum monthly evapotranspiration occurs in mid-summer, with an average of mm, while in winter the monthly evapotranspiration averages below 5mm. 4. SAMPLE LOCATION AND BORE DATA Water samples were collected from wells, springs, and rivers by GNS Science. Sampling sites were selected to represent the whole catchment, including historical sampling sites for age tracers from between to provide time series tritium data to reduce ambiguity in age interpretation and constrain mixing in Lumped Parameter models. Details of the sampling locations shown in Figure 1 are listed in Table 2 and include well identification, location, construction details, water level, aquifer lithology and condition, and land use around the sampling site. These details were supplied by HBRC. GNS Science Report 212/23 7

14 DRAFT 212 Table 2 Well identification, location, construction details, water level, aquifer lithology and condition, and land-use information. Data provided by HBRC. Coordinates are NZ Map Grid (NZGD1949 datum). Bore depth is depth to bottom of well from ground surface. Wlbg is water level below the ground surface. Bore No. # Address E N Bore Top Bottom Depth Screen Screen Wlbg m m Aquifer Lithology Aquifer Condition S.H. 2, WAIPUKURAU (L/C) P.O, BOX Gravels Unconfined ORUAWHARA RD, TAKAPAU (L/C) Confined grazing Burnside Road Gravels Confined dairy S.H. 5, ONGA ONGA. (L/C) Gravels Confined dairy WAKARARA RD,ONGA ONG (L/C) Gravels Unconfined apple orchard TIKOKINO RD, TIKOKINO (L/C) conf/artesian dairy ASHCOTT RD, (L/C), WAIPUKURAU Gravels Confined HOBIN RD, RUATANIWHA (L/C) Confined sheep beef grazing PAGET RD, TAKAPAU (L/C) c.5 Gravels Confined dairy WAKARARA RD, ONGA ONGA (L/C) c. conf/artesian apple orchard TE PAPA, S.H.5, ONGAONGA (No.2) (L/C) Gravels Unconfined apple orchard BUTLER ROAD, TIKOKINO Unknown Unknown TUKITUKI ROAD (L/C) Unconfined WAIPAWA/ 1345 TIKOKINO ROAD Unknown Unknown grazing SWAMP ROAD, ONGA ONGA conf/artesian apple orchard S.H. 2, TAKAPAU c. Gravels Confined pastural grazing STATE HIGHWAY 5, TAKAPAU Unknown Unknown grazing BURNSIDE ROAD, TAKAPAU Gravels Confined dairy STOCKADE RD RIVER RESERVE, WAIPAWA Gravels Unconfined dairy STATION ROAD, TAKAPAU Limestone Confined grazing FRASER RD TAKAPAU (L/C) Confined Gravels Confined grazing FRASER RD TAKAPAU (L/C) Gravels Confined freezer works MAKARORO ROAD, TIKOKINO Unconfined TUIVALE, MAKARORO RD, TIKOKINO Confined sheep grazing WaipR 8 Waipawa River SH N/A TukitR 9 Tukituki River SH N/A TukitR 11 Tukituki River Ongaonga Road N/A DC Spring 14 Drainage channel Spring N/A dairy L Spring 15 Limestone Spring Limestone N/A dairy, grazing landuse 5. HYDROCHEMISTRY Chemistry data for the Ruataniwha Basin sites are compiled in Table 3. Samples from twenty-six sites were collected for this study by HBRC. Site 1376 has times series data, because this well is sampled quarterly as part of the National Groundwater Monitoring Programme run by GNS Science (Daughney & Reeves, 25). Sampling methods and analytical details for hydrochemistry are described in Daughney et al. (21). GNS Science Report 212/23 8

15 Table 3 Chemistry data for Ruataniwha Basin sites. Unusual concentrations for groundwater samples are highlighted: light green low, pink high, red very high. Total concentrations of anions and cations and charge balance error (CBE) calculated according to Freeze and Cherry (1979). Conductivity and ph are lab values, dissolved oxygen (DO) are field values. Data are from samples collected between February and April 29, except 222, 372, 4694, 472 (samples collected 16/2/11 after sufficient purging). Results of sample 6722 are shaded grey because this well was not re-sampled to obtain water after purging. Data for bore 1942 are median values from quarterly samples taken between 1998 and 27. Site Cluster Ca K Na Mg Cl SO 4 HCO 3 SiO 2 Fe Mn NH 3-N NO 3-N temp DO CH 4 Cond ph TDS mgl -1 mgl -1 mgl -1 mgl -1 mgl -1 mgl -1 mgl -1 mgl -1 mgl -1 mgl -1 mgl -1 mgl -1 C mgl -1 μmollkg - 1 μscm -1 mgl A < A B < A < B < B < B A < A A < A < A < <.2.5 < R A < A < Waipawa R < Tukituki R. SH < Tukituki R < Drainage Spr. 2B < Limestone Spr. 2B < GNS Science Report 212/23 9

16 Site Anions Cations CBE Al As B Cd Co Cr Cu F Li Mo Ni NO 2 Pb Sb Sn V Zn meql -1 meql -1 mgl -1 mgl -1 mgl -1 mgl -1 mgl -1 mgl -1 mgl -1 mgl -1 mgl -1 mgl -1 mgl -1 mgl -1 mgl -1 mgl -1 mgl -1 mgl -1 mgl % % % % % % % % % % % % % % % % % % % % % % Waipawa R % Tukituki R. SH % Tukituki R % Drainage Spr % Limestone Spr % GNS Science Report 212/23 1

17 5.1. National context The hydrochemistry of the samples collected for this study can be understood in a national context by comparison to data compiled from all areas of New Zealand. In this study, groundwater quality data obtained from over 1 sites collected as part of state of the environment (SOE) monitoring programmes run by regional authorities and compiled by the New Zealand Ministry for the Environment (27) have been used. Special characteristics in the groundwater chemistry of sites from this study are identified from comparison to the national SOE dataset. In particular, many sites from Ruataniwha aquifers have low concentrations of sodium, chloride and sulphate with respect to the SOE dataset. Some sites have high concentrations of silica, while others have extremely low silica concentrations. Many sites have unusually high ph values. In Table 3, sites with particularly low or high concentrations of any analyte are highlighted. Concentrations below the SOE 25 th and 5 th percentiles are highlighted in light green or dark green respectively; while high concentrations above the SOE 75 th and 95 th percentiles are highlighted in pink and red respectively. Notable characteristics of the groundwater chemistry from the study sites include: Conductivity values range from 95 to 436 μscm -1. Nine sites, including the three river sites, have conductivity values below the SOE 25 th percentile of 145 μscm -1. Two bores (1381, 4722) have conductivity values above the SOE 75 th percentile (371 μscm -1 ). ph values for the majority of sites are unusually high. Seventeen sites, including the three river sites, have ph values above the SOE 75 th percentile of 7.2. Of these, six sites have ph values above the SOE 5 th percentile (7.9). Five sites have silica concentrations below the SOE 25 th percentile (13.5 mgl -1 ). This includes the three river sites, which have silica concentrations between 9 mgl -1 and 11 mgl -1. Twelve sites have silica concentrations above the SOE 75 th percentile (29.5 mgl -1 ), ranging from 31 to 69 mgl -1. Seventeen sites have chloride concentrations below the SOE 25 th percentile (7.3 mgl -1 ), while nine sites have sulphate concentrations below the SOE 25 th percentile (3 mgl -1 ). Ten sites have NH 3 -N concentrations above the SOE 75 th percentile, ranging from.16 to 1.8 mgl -1. In addition, six of these bores have elevated iron concentrations which range between.45 and 1.4 mgl -1, in comparison to the SOE 75 th percentile value of.23 mgl -1. Five bores with elevated NH 3 -N also have high manganese concentrations. These five and an additional two sites have manganese concentrations above the SOE 75 th percentile (.24 mgl -1 ). The high ammonia, iron and manganese concentrations are correlated to low nitrate and low dissolved oxygen concentrations because of the low redox state of the groundwater from these sites. Sites 1452 and 1558 have elevated nitrate-nitrogen concentrations of 6.1 and 4.6 mgl -1 respectively. For comparison, the SOE 75 th percentile for nitrate is 4.4 mgl -1. These elevated levels are likely to have anthropogenic origins and indicate the predominance of rainfall recharge to these bores. GNS Science Report 212/23 11

18 Site 1381 has potassium, sodium, magnesium, and bicarbonate concentrations above the respective national SOE 75 th percentiles. Sites 1376 and 3852 have similarly high sodium concentrations. Site 472 has magnesium concentration of 4 mgl -1, which is above the national SOE 95 th percentile, and is accompanied by high concentrations of calcium, bicarbonate and sulphate, all of which are above the respective SOE 75 th percentiles. Sites 1381 and 6719 have elevated boron concentrations greater than the national SOE 75 th percentile, along with site Variations in hydrochemistry within the study area An assessment of the variations in hydrochemistry within the study area has been carried out using hierarchical cluster analysis (HCA), following Daughney and Reeves (25), using logtransformed and normalised levels of the major ions calcium, magnesium, sodium, potassium, bicarbonate, chloride, and sulphate, conductivity, silica and ph (not transformed) for 26 sites. Ions that reflect the redox state of the groundwater (nitrate, ammonia, iron and manganese) were omitted from the analysis to prevent bias towards redox state. Fluorine was also omitted due to missing data for some sites. Hierarchical cluster analysis results are displayed as a dendrogram (Figure 4). Each vertical blue line ends at a single sample site. Horizontal blue lines join groups of sites. The similarity between sites and groups of sites is indicated by the height up the y-axis (Distance) of the respective connecting horizontal line. Low horizontal lines join sites with the most similar chemistry. For example, in Figure 4, the cluster analysis indicates that sites 1452, 1944, 1558 and 1655 have very similar hydrochemistry, because they are joined by lines that are low on the y-axis (Distance), whereas these sites have very different chemistry from sites 1376, 3426, 6719 and The analysis reveals 4 major groupings of sites with similar chemistry (Figures 4 and 5). Clusters 1 and 3 have sites with chemistry that is distinct from other groups, while clusters 2A and 2B are more closely related. Cluster 2A can be further divided into two sub-clusters. The geographic distribution of the clusters is shown in Figure 6. Hydrochemical features of each cluster are outlined below. Cluster 3, comprising the river sites and two bores, has groundwater chemistry that differs most from all the other clusters. The two bores, one a shallow bore of 12 m (376) and the other a slightly deeper bore of 44 m (4694), are both located adjacent to the Waipawa River. Cluster 3 is characterised by low concentrations of most solutes, especially silica and chloride. Bicarbonate, although concentrations are low, is the dominant anion, while calcium is the dominant cation. Conductivity is low, reflecting a low total ionic loading. The relationship of groundwater from bore 376 to the Waipawa River is confirmed by stable isotope data (Section 6). Cluster 1 (Sites 6719, 3852, 3426, and 1376) also has very distinct groundwater chemistry. These sites are situated in the south of the study area, with two shallow bores of around 2 m depth and two deeper bores with depths of 88 m and 121 m. Conductivity for cluster 1 GNS Science Report 212/23 12

19 was the highest of all clusters, accompanied by correspondingly higher concentrations of bicarbonate, chloride and sodium. All sites in this cluster have anoxic, reduced groundwater, with low dissolved oxygen and nitrate concentrations, and high concentrations of ammonium, iron and manganese. The groundwater from several of these bores also contains dissolved methane. The sites in this cluster are situated in the south of the study area (Figure 6) Distance Drainage ch Spr Limestone Spr 376 Tukituki R. SH5 Tukituki R. W.O Rd 4694 Waipawa R.SH5 1 2A1 2A2 2B 3 Figure 4 Dendrogram produced by hierarchical cluster analysis. The sites are grouped into four main clusters at the chosen threshold indicated by the dashed line. Cluster 2A is further partitioned into two sub-clusters (2A1, 2A2). GNS Science Report 212/23 13

20 Cluster 1 2A1 2A B R ND (6722) Ca %meq.l -1 Cl 1 Figure 5 Piper diagram showing the variation of major ion chemistry in the Ruataniwha samples. The left and right triangles show the major cation and anion ratios on an equivalence basis respectively, and the centre diamond plots projections based on the two triangular plots. Sites are grouped into clusters assigned by hierarchical cluster analysis. R indicates residual bores not assigned into a cluster. Open symbols show the very unusual chemistry of four sites after sampling without sufficient purging. The arrow points to the value after sufficient purging. GNS Science Report 212/23 14

21 Figure 6 Geographic distribution of sites assigned to hierarchical cluster analysis-defined clusters. Clusters 2A and 2B have solute loads and conductivities intermediate between clusters 1 and 3. These sites are located in the centre and north of the study area. Cluster 2A bores range in depth from 25 m to 142 m. This cluster is a mix of Ca-HCO 3 and Na HCO 3 dominant groundwaters and can be further divided into two sub-clusters, 2A1 and 2A2. Subcluster 2A1 has higher conductivity calcium and bicarbonate but lower sodium than subcluster 2A2. Groundwater from sub-cluster 2A2 bores is low in dissolved oxygen and has very little nitrate, but slightly elevated concentrations of dissolved iron and manganese, while groundwater from 2A2 is oxic and has negligible dissolved iron and manganese, and low but significantly higher nitrate than 2A1. GNS Science Report 212/23 15

22 Cluster 2B, consisting of shallower bores and springs, has slightly higher conductivity but lower silica than cluster 2A. Concentrations of calcium, chloride and bicarbonate are higher in this cluster than in cluster 2A. There are four bores in this cluster (1452, 1558, 1655 and 1944), of which three are shallow bores between 12 and 22 m deep, and one a deeper bore of 55 m. However, the deeper bore 1452 is thought to penetrate the same shallow unconfined aquifer system as the other three bores (Undereiner et al., 29). Two of the bores (1452, 1588) have elevated concentrations of nitrate. The other two bores have low nitrate concentrations, but are anoxic. Measured concentrations of the dissolved gases Ar and N 2 indicate a component of excess dissolved N 2 that is likely to be derived from denitrification (Section 3). Therefore, all four bores in this cluster indicate an anthropogenic influence. The high nitrate, accompanied by high chloride concentrations, indicates significant rainfall-derived recharge. The two springs in the cluster do not show the same anthropogenic influence. Site 472 is a residual in the hierarchical cluster analysis, with groundwater chemistry unrelated to the other sites sampled. This site had very high conductivity and much higher calcium, magnesium and bicarbonate concentrations than the other sites Hydrochemistry variation and trends over time For a number of the wells, HBRC provided times series hydrochemistry data reaching back 15 years. Some of these wells have relatively constant hydrochemistry, for example well 6719 (Figure 7). In contrast, other wells show highly variable hydrochemistry or trends over time, for example, wells 1518, 1558, and 6719 (Figures 8 1). All of these are relatively deep wells (21 88 m) with reasonably old water ( y). Therefore this variability is unlikely to be caused by surface processes (for example seasonal changes) but more likely indicates a highly fractured structure of the water-bearing layers. Pockets of water-bearing gravels may co-exist next to each other with different sources of the water, and depending on the amount of abstraction and/or recharge, the different reservoirs are activated. Well 1944 and 6719 for example have very high soluble phosphate and ammonia. GNS Science Report 212/23 16

23 2 Well 6719 mg/l 1 NH3 P (soluble reactive) mg/l 2 1 Graph 1 NO3 Fe Mn SO4 K mg/l Bicarb TotAlk HDT TDS 5 4 mg/l Na Cl Ca Mg temp SiO2 Cond (us/cm) WL (m) 25 2 ph WL Trit sampling well6719.grf Figure 7 Water level and hydrochemistry trends of well GNS Science Report 212/23 17

24 Well 1944 also shows high variability in Fe concentration and indicates a switch between different reservoirs having different redox conditions. Increasing sulphur indicates increasing land-use impact over time, or a depleting reservoir with lower SO 4. 1 Well 1944 mg/l NH3 P (soluble reactive) mg/l 15 1 NO3 Fe Mn SO4 K 5 mg/l 15 1 Bicarb TotAlk HDT TDS 5 5 mg/l Na Cl Ca Mg temp SiO2 Cond (us/cm) ph 7 WL (m) -5-1 WL Trit sampling well1944.grf Figure 8 Water level and hydrochemistry trends of well GNS Science Report 212/23 18

25 Well 1518 also shows high variability in Fe, Mn, NH 3, P, and SO 4, and indicates a switch between reservoirs with different redox conditions. When Fe is high, Mn and NH 3 are also high, indicating a source from a reduced reservoir. The water from the oxic reservoir has significantly increased Si and P concentrations..2 Well 1518 mg/l.1 NH3 P (soluble reactive) mg/l NO3 Fe Mn SO4 K 1 mg/l 1 5 Bicarb TotAlk HDT TDS 5 mg/l Cond (us/cm) ph WL (m) Na Cl Ca Mg temp SiO2 WL Trit sampling well1518.grf 21 Figure 9 Water level and hydrochemistry trends of well GNS Science Report 212/23 19

26 Well 1558 shows stable oxic conditions, but increasing SO 4, which indicates increasing landuse impact. Slight increases in Na, Cl, Mg and Ca also point towards increasing impact from fertilizer..15 Well 1558 mg/l.1.5 mg/l NH3 P (soluble reactive) NO3 Fe Mn SO4 K 2 mg/l 15 1 Bicarb TotAlk HDT TDS 5 3 mg/l 2 1 Na Cl Ca Mg temp SiO2 Cond (us/cm) ph -4 WL (m) WL Trit sampling well1558.grf 21 Figure 1 Water level and hydrochemistry trends of well GNS Science Report 212/23 2

27 6. STABLE ISOTOPES AND RADON AS INDICATORS OF RECHARGE SOURCE The stable isotopes of the water molecules and radon have contrasting signatures in riverand rain-recharged groundwater and may be used to indicate the recharge source. Stable isotope ratios 2/1 H and 18/16 O in groundwater samples were analysed to assess if there was a characteristic pattern in these ratios that could be used to distinguish between recharge derived from rivers with higher altitude catchments and recharge derived from local rainfall. The ratios are expressed as δ values where: δ 18 O VSMOW ( ) = [( 18 O/ 16 O) sample /( 18 O/ 16 O) VSMOW - 1] x 1 δ 2 H is calculated similarly. The δ values represent the difference in parts per thousand between isotope ratios in water relative to those in the standard (Vienna Standard Mean Ocean Water or V-SMOW). Stable isotope analytical results for Ruataniwha groundwater sample sites are listed in Table 4 and are illustrated in Figure 11. Table 4 Values for field parameters, radon, and stable isotopes. Data shaded in grey are from samples with insufficient purging of the well. Bore No. # Sample Date temp [oc] cond. [us/cm] ph DO [mg/l] Rn [Bq/L] ± δ 18 O ± δd ± /7/ /3/ /2/ /2/ /2/ /2/ /1/ /2/ /2/ /2/ /3/ /2/ /5/ /2/ /2/ /3/ /2/ /3/ /2/ /2/ /2/ /2/ /2/ /2/ /3/ /3/ /3/ /1/ /3/ WaipR 8 2/2/ TukitR 9 2/2/ TukitR 11 3/2/ DC Spring 14 3/2/ DC Spring 14 1/3/ L Spring 15 3/2/ L Spring 15 1/3/ GNS Science Report 212/23 21

28 -4 May-1-42 Apr-1-44 Tukituki R δ 2 H [ ] Aug-9 Drainage Spr. Mar-11 Sep-9 Jun-9 Limestone Spr. Mar-11 Nov-9 Mar Feb Limestone Spr. Apr-9 Drainage Spr. Apr Dec-9 Jan Waipawa R Jul-1 Jun-1 Oct δ 18 O [ ] Ruataniwha samples D=8x18O+1 D=8x18O+13 Makaroro R. Figure 11 Plot of δ 18 O against δ 2 H for Ruataniwha sites. Data for the Makaroro River, collected at monthly intervals from June 29 to Feb 21, are also shown. The blue dashed line is the local meteoric line (δ 2 H=8xδ 18 O+13) observed for river water and rainfall in the west of New Zealand, while the red dashed line is the meteoric line (δ 2 H=8xδ 18 O+1) observed for the eastern areas of New Zealand (Stewart & Taylor, 1981). Error bars are not shown for clarity, but the reported analytical precision is.1 for δ 18 O and 1. for δ 2 H. δ 18 O values for the sample sites ranged from -8.1 to -6.6 and δ 2 H values between -5 and -44. δ 18 O values for the two rivers were -7.2 and -6.9, and δ 2 H are -46 and -44, for the Waipawa and Tukituki Rivers respectively. The river sites are located in the upper area of the basin. Also illustrated in Figure 11 are isotope data for the Makaroro River. This river, situated 1.5 km west of sampling site 115 above the confluence with the Waipawa River, has had samples collected at monthly intervals as part of a national isoscape project. Data are available so far for the period June 29 to May 21. Isotopic values reflect a predominance of north-westerly rainfall in this catchment and plot nearer to the meteoric line for 2 H and 18 O that has been observed in westerly rainfall and river water in New Zealand (Stewart & Morgenstern, 21). The isotopic values for this river are generally more negative than those of the study sites due to the higher altitude of the catchment. δ 18 O values varied between -8.2 and -6.3 and δ 2 H values between -53 and -41, showing a seasonal variation that has been little damped and therefore implies a short residence time in the catchment. As the Makaroro River is a tributary of the Waipawa, it might be expected that the Waipawa River would have a similar isotopic signature. However a sample taken from the site Waipawa River@State Highway 5 for this study had isotope values at the positive end of the Makaroro river data range, which may indicate seasonal variation for the sampling date in April 29. GNS Science Report 212/23 22

29 Only a few bore sites that were sampled show an isotopic signature similar to the Makaroro / Waipawa Rivers. These are bores 376, 143, 222, 372, 472 and the Limestone and Drainage channel springs. The springs show considerable variation between two separate sampling events, which may indicate seasonal variability and therefore a young (<2 y) groundwater age. Stable isotope values for most other sites plot nearer to the meteoric line for 2 H and 18 O observed in rainfall and river water in eastern areas of New Zealand; the difference to the western area has been attributed to rainout effects and differences in the direction of the prevailing weather (Stewart & Taylor, 1981). This infers that for these sites, the major recharge source is local rainfall, or smaller streams and rivers with catchments on the eastern side of the main ranges which catch more eastern New Zealand type precipitation. Figure 12 depicts the relationship between δ 18 O values and chloride concentrations. While there is no clear pattern, a distinction can be made between sites with low chloride concentrations and slightly more negative isotopic values, implying river-derived recharge; and sites with more positive isotopic values, possibly accompanied by higher chloride concentrations, implying higher proportions of rainfall-derived recharge Cl [mgl -1 ] Drainage Spr. Limestone Spr Waipawa R Tukituki R δ 18 O [ ] Figure 12 Plot of δ 18 O against chloride. Although there is some overlap, sites can be categorised into predominantly river-water derived recharge (blue box) or rainfall-derived recharge (green box). GNS Science Report 212/23 23

30 Radon data are listed in Table 4. All radon concentrations are relatively low. Radon concentrations for river water are close to zero, as expected. The Drainage channel spring has a radon concentration close zero, which may indicate very young river water. The Limestone spring has a similar low radon concentration, but this may be caused by degassing. This is supported by Ar and N 2 data. The water probably had air contact upstream of the sampling point, but the flow path upstream was not accessible. A number of well waters also have zero radon concentration, which is probably caused by adsorption to organic material, such as in peat layers. Peat layers are sparsely indicated in the bore logs. The radon data does not show significant differences to provide conclusive characterisation of recharge sources. 7. RECHARGE TEMPERATURES AND EXCESS AIR CONCENTRATIONS AS INDICATORS OF RECHARGE SOURCE Recharge temperatures and a component of dissolved excess air have been derived from dissolved argon and nitrogen concentrations using the total dissolution model of Heaton and Vogel (1981). In this model, small bubbles of air entrapped in soil pores are completely dissolved into the groundwater under favourable recharge conditions, thus forming an excess air component. Calculated recharge temperatures for the groundwater sites range from 5.5 C to 17.9 C (Table 5). In comparison, the mean annual air temperature of the study area over the period from 1971 to 2 was 12.1 C, measured at the climate station at Makaretu, with elevation of 335 m above mean sea level (NIWA National Climate Database 21). Temperatures derived from the gas concentrations in the three surface water samples were high, ranging from 15.2 C to 17.4 C, and were within analytical error of the water temperatures observed at the time of sampling. GNS Science Report 212/23 24

31 Table 5 Dissolved argon and nitrogen concentrations and derived variables. Ar, N2 and excess air concentrations are expressed in ml of the respective gas at standard temperature and pressure (STP =273.15K, kPa) per kg of water. Specified errors are one standard analytical uncertainty. Negative values that are significantly less than zero indicate sample degassing. Recharge temperatures and excess air concentrations for sites 1655 and 1944 (pink) may actually be lower than indicated, due to an excess nitrogen component (see main text). Ar ± N 2 ± temp ± excess air ± Site ml(stp).kg -1 ml(stp).kg -1 C ml(stp).kg Drainage channel spring Limestone spring Tukituki River SH Tukituki River, Parsons Rd Waipawa River SH GNS Science Report 212/23 25

32 Figure 13 shows the argon and nitrogen concentrations in groundwater from the sample sites. Samples within the grid are able to have recharge temperatures and excess air concentrations calculated. On the basis of their positions on the grid, the sites can be roughly grouped as follows: Values for sites 1518, 376 and Limestone spring plot to the left and outside of the grid, indicating that these samples have undergone degassing. While approximate corrections can be made for these samples to allow calculation of recharge temperatures (Pankow, 1986), the temperatures derived in such a fashion should be treated with caution. Sites which have low concentrations of excess air (<2 ml (STP)kg -1 ) and recharge temperatures close to the mean annual air temperature ( ): this group consists of bores 143, 243, and the Drainage channel spring. Sites within this group may be expected to have a high component of river-derived recharge. River recharge will not undergo the same processes as rainfall recharge which can cause high concentrations of excess air, and the river recharge is more constant than rainfall. For sites where the temperature signal of the river has been smoothed out over time (i.e. groundwater with an age greater than a few years), the recharge temperature may be expected to be close to the mean annual temperature. Sites which have higher concentrations of excess air ( ml(stp)kg -1 ) and lower recharge temperatures ( C): this group includes the sites 1381, 142, 1452, 2579, 314, 3426, 3852, 6719, 1942 and 115. The lower recharge temperatures maybe indicative of several different processes: o o In recharge areas where the unsaturated zone is thin, the temperature above the water table may respond to seasonal effects (Busenberg & Plummer, 2). Low recharge temperatures may indicate a predominance of recharge occurring during winter. Alternatively the unusually low recharge temperatures and high concentrations of dissolved excess air may indicate that the dissolved Ar and N 2 concentrations in groundwater from these bores do not fit the total dissolution model of excess air and may be better interpreted using an alternative model of excess air formation, such as the closed-system equilibration model (Aeschbach-Hertig et al., 2). However, for accurate interpretation, the closed-system equilibration model requires measurement of extra dissolved gases such as neon. GNS Science Report 212/23 26

33 .48 Temperature C C 1 C C Ar [ml(stp).kg -1 ] Limestone spr. Tukituki R. Tukituki R. Waipawa R 1518 Drainage spr excess N 2 excess air ml(stp).kg C N 2 [ml(stp).kg -1 ] Figure 13 Plot of nitrogen versus argon concentrations, normalised to sea level. The positions of the points within the grid indicate the recharge temperatures and excess air concentrations of the samples. The bold line on the left of the grid is for gas concentrations in water in equilibrium with the atmosphere. Samples from sites 1518, 376 and Limestone spring have been degassed, probably during sampling, and plot to the left of this line. The green dashed line indicates the mean annual air temperature for the study area. The red arrows indicate possible amounts of excess nitrogen affecting the samples from sites 1655 and Regardless of which of the above two scenarios is correct, these sites may be expected to have a component of rainfall-derived recharge, as indicated by the higher excess air concentrations. Bores 1655 and 1944 have high apparent recharge temperatures and high concentrations of excess air. This could be indicative of two different processes: o o The groundwater from these bores is derived from recharge areas with shallow unsaturated zones and a seasonal effect is being seen. This would imply that the recharge has occurred in the hotter period of the year, i.e. summer. The groundwater from these bores may have excess nitrogen from denitrification. Hierarchical cluster analysis (Section 5.2) indicates that these two sites have similar groundwater to two bores with high nitrate concentrations. As both 1655 and 1944 have anoxic, reduced groundwater, as indicated by their low dissolved oxygen concentrations and elevated iron, manganese and ammonium concentrations, this scenario seems most likely. Figure 9 illustrates the probable amounts of excess nitrogen for these sites under this scenario (red arrows), assuming recharge temperatures that are the same as the mean annual air temperature of 12.1 C. This gives excess nitrogen concentrations of 2.1 ml(stp)kg - 1 for bore 1655 and 1.8 ml(stp)kg -1 for If all of this excess nitrogen is derived from the denitrification GNS Science Report 212/23 27

34 of nitrate, then the original nitrate concentrations for the groundwater from these bores would be 2.2 mgl -1 and 2.6 mgl -1 respectively. Both scenarios imply a significant component of rainfall recharge. As both excess air concentrations and chloride concentrations can be linked to the occurrence or lack of rainfall-derived recharge, we might expect to see a relationship between these two parameters. Figure 14 illustrates this relationship. It can be seen that a weak relationship exists, wherein sites with low excess air concentrations have predominantly lower chloride concentrations, and sites with higher excess air concentrations have mainly higher chloride concentrations Chloride [mgl -1 ] 8 6 Drainage Spr Tukituki R. SH5 4 Waipawa R. Tukituki R dissolved excess air [ml(stp)kg -1 ] Figure 14 Plot of excess air against chloride concentrations. Two groups of sites are shown, those within the blue oval are thought to be dominated by river-derived recharge, and those within the green oval by rainfall-derived recharge. Excess air concentrations for sites 1655 and 1944 have been corrected for excess nitrogen. 8. GROUNDWATER DATING The age tracer compounds tritium, CFCs, SF 6 were measured in the Ruataniwha Basin in groundwater wells, springs, and rivers. Analytical techniques are described in Morgenstern and Daughney (212), and van der Raaij (23). Analytical results in Table 6 include tritium results from previous studies. GNS Science Report 212/23 28

35 Table 6 Tritium, SF 6, and CFC results, together with groundwater age parameters. MRT is the mean residence time in years. The parameter f is the fraction of exponential (mixed) flow in the total flow volume which could be determined for tritium time series data, otherwise f is estimated. Data with grey background are from samples taken without sufficient purging of the well and may be compromised. When age interpretation is ambiguous, data in bold are the more realistic age distribution parameters. Bore No. # T No Sample Date TR ± Sample Date SF6 pptv ± CFC11 pptv ± CFC1 2 pptv ± f estim f determ MRT y THB124 7/7/ /3/ > THB172 1/3/ /3/ > THB153 2/2/ /2/ THB154 2/2/ /2/ > THB165 3/2/ /2/ THB166 3/2/ /2/ THB146 2/1/ /3/ ? 3/2/ /2/ THB149 2/2/ /2/ THB155 2/2/ /2/ THB17 1/3/ /3/ THB61 21/5/ > THB158 2/2/ /2/ THB167 3/2/ /2/ TM643 9/3/ /2/ > THB15 2/2/ /2/ THB169 1/3/ /3/ > THB52 11/2/ /2/ > THB161 3/2/ /2/ /4 <26/ THB151 2/2/ /2/ TM642 9/3/ THB173 1/3/ /3/ THB168 1/3/ /3/ THB145 2/1/ THB171 1/3/ /3/ WaipR 8 THB156 2/2/ /2/ TukitR 9 THB157 2/2/ /2/ TukitR 11 THB159 3/2/ /2/ DC Spring 14 THB162 3/2/ /2/ /2 1/47 L Spring 15 THB163 3/2/ /2/ /2 1/ Methodology of groundwater dating The methodology of age interpretation of the age tracer data in groundwater is described in Morgenstern and Daughney (212), and in stream and river water in Morgenstern et al. (21). The convolution integral relates measured tracer concentrations to historical tracer input (Figure 15). Groundwater at its discharge point is a mixture of water from short and long flow lines, and therefore has a distribution of ages rather than one age. One parameter of the age distribution is the mean residence time (MRT), and the other parameter describes mixing. Various response functions describe the distribution of ages within the water sample (Maloszewski & Zuber, 1982, 1991; Zuber et al., 25). The two most commonly employed response functions are the dispersion model and the exponential piston flow model. The exponential piston flow model is a combination of the piston flow model, which assumes piston flow with minimal mixing of water from different flow lines at the discharge point, as might occur in a narrow confined aquifer, and the exponential model, which assumes that GNS Science Report 212/23 29

36 transit times are exponentially distributed at the groundwater discharge point, as might occur for mixing of stratified groundwater at the discharge point in unconfined aquifers. The various response functions are described in Zuber et al. (25). We used the exponential piston flow model for the age interpretation of the Ruataniwha age tracer data because the age distribution of this model is realistic, as demonstrated by excellent matches of the model output to tritium time series data throughout New Zealand, including the Ruataniwha Plains. The fraction of exponential flow within the total flow volume is given as f (in %) in Table Tritium 6 TU CFC & SF6 concentration [pptv] Figure 15 Historic age tracer input functions. The tritium input function is based on tritium concentrations measured monthly since 196 at Kaitoke, near Wellington, New Zealand (Morgenstern & Taylor, 29), with a scaling factor.9 to account for difference in latitude (Morgenstern et al., 21). No correction was made for seasonal variation of recharge, because under New Zealand climatic conditions tritium input does not change significantly (Morgenstern et al., 21). CFC input functions are based on measured data from Cape Grim, Australia, and other southern hemisphere sites (Cunnold et al., 1997; Prinn et al., 2). CFC concentrations prior to 198 have been reconstructed (Walker et al., 2). The SF 6 curve is based on measured and reconstructed data (Maiss & Brenninkmeijer, 1998; Thompson et al., 24) Interpretation of tritium time series and gas tracer data This investigation involved analysis of tritium, CFC and SF 6 concentrations in samples collected from the Ruataniwha Basin sites in 29. Age interpretations also rely on previous tritium measurements made as part of earlier investigations. GNS Science Report 212/23 3

37 To obtain the best fitting age distribution parameters for the measured tracer data for those sites at which time series tracer data were available, the variables MRT and f were simultaneously optimized by minimization of the root mean square error between the predicted and measured tracer concentrations. For other sites, f was estimated on the basis of aquifer lithology and time-series tracer measurements made at other sites in New Zealand with similar aquifer characteristics and with similar well construction (depth, width of well screen, aquifer confinement). The derived MRT values have an associated uncertainty of about 1 4 years. The magnitude of this uncertainty depends on groundwater age and also on the range of time-series data available, where sites without time-series tracer measurements have a higher uncertainty in their MRT. This uncertainty is typically one year for young waters having a MRT<5 years, increasing to 4 years for a MRT of 8 years. Wells 1452 and 1518, with long tritium time series data over 26 years, and wells 222, 1944, 243, 1558 and 376, with time series data over more than 15 years, allow for robust age distributions. MRT s are listed in Table 6, and shown in Figure 16. Figure 16 Groundwater MRT s in years, together with bore depth. GNS Science Report 212/23 31

38 9. DISCUSSION Most groundwaters sampled across the Ruataniwha Basin have old water, with MRT > 25 years. Only one well (376) has younger water and it is likely to be linked to the river via a fast flow path such as paleo river channels. The old age of most of the waters indicates that these groundwaters are not directly linked to the surface water via fast flow paths. Figure 17a shows that in the Ruataniwha Basin there is a general correlation between groundwater age and well depth, with increasing age in deeper wells. However, the correlation is poor and indicates a highly heterogeneous aquifer. In comparison, the Linkwater groundwater system in Marlborough (Figure 17b) has a better correlation between MRT and depth for rain recharge, due to a more homogenous aquifer structure. The MRT versus depth correlation in the Ruataniwha Basin may also be disturbed due to the closed nature of the basin, with all groundwaters having to discharge from the basin via surface flow, which causes upwelling groundwater flow near the discharge area of the basin. This is indicated by the extreme values in Figure 17a for well 1376 (#29), which has relatively old water for the depth, indicating that this well is located in the upwelling area, while well 314 (#16) and well 372 (#21) are not in the upwelling area and have old water at great depth. Recharge rate can be deduced from the vertical flow velocity (Cook & Herczeg, 1999). As the Ruataniwha depth versus age relationship is likely to be biased because of upwelling groundwater flow, which will shift the values affected by upwelling in Figure 17a to the right, ignoring the values on the right can give an estimate of average recharge rate in the basin. This age-depth relationship is shown in Figure 17a as a black line. The values with a yellow background in Figure 17 were excluded for this relationship (see below). The estimated vertical flow velocity is 155 m / 2 y =.78 m/y. Assuming an average porosity through the basin of.25, the groundwater recharge rate calculated from the vertical flow velocity is.19 m/y. In contrast, the data on the left of Figure 17a with yellow highlight (wells 376, 1452, 243, and 222) are all in the vicinity of the lower Waipawa River, downstream from a losing stretch of the river. The excellent age-depth relationship of these data (yellow line) indicates that in this area the age-depth relationship is not biased by groundwater upwelling. The vertical flow velocity of 15 m / 9 y = 1.7 m/y correlates to a recharge rate of.42 m/y. This is significantly higher than the rate of.19 m/y calculated for the other parts of the basin and indicates significant river recharge in addition to the recharge by rain. GNS Science Report 212/23 32

39 Ruataniwha basin a Linkwater - Marlborough b Depth (m) oxic anoxic unknown Depth [m] stream recharge rain recharge MRT [y] MRT [y] Figure 17 Depth versus MRT for the Ruataniwha Basin (a) and Linkwater-Malborough (b). Labels refer to ID numbers in tables for the Ruataniwha Basin. The Linkwater example (Figure 17b) shows the contrasting pattern between rain recharge, and fast recharge along connected gravel aquifers from the stream. This pattern is not observed in the Ruataniwha Basin (Figure 17a). Figure 18 shows the redox-sensitive parameters Dissolved Oxygen, Fe, Mn, together with reaction products of anoxic processes CH 4 and NH 3, and B, which often is mobilised in anoxic processes. Dissolved oxygen (DO) in groundwater is consumed by oxidation of organic matter or other electron donors (e.g., pyrite) in the aquifer matrix. Reduction of oxygen is energetically the most favourable reaction that micro-organisms use, with the result that other reduction reactions (e.g., denitrification) typically do not occur until most of the dissolved oxygen has been consumed. These reactions take time and it is expected that old waters become increasingly anoxic. The Ruataniwha groundwaters (Figure 18) show such a trend of depletion in oxygen with increasing residence time. For water older than 8 years the DO is completely depleted, and presence of increasing concentrations of Fe, Mn, CH 4, NH 3, and B indicate increasing strength of reduced conditions. GNS Science Report 212/23 33

40 oxic anoxic unknown DO (mg/l) B (mg/l) MRT [y] MRT [y] Fe (mg/l) 1 23 Mn (mg/l) MRT [y] MRT [y] CH4 (umol/l) 4 NH3 (mg/l) MRT [y] MRT [y] Figure 18 Redox sensitive parameters DO, Fe, Mn, CH 4, NH 3, and B versus MRT in years. GNS Science Report 212/23 34

41 Major ion concentration data and ph can complement groundwater age interpretation because they increase with groundwater age due to mineral dissolution occurring during water-rock interaction. Although relationships between groundwater age and ion concentrations can be complicated, increasing major ion concentrations with groundwater age were found in several previous investigations in New Zealand (Morgenstern et al., 24, 29) with very good correlations between the MRT of the water in the groundwater system and major chemistry concentration of the groundwater. Five of the groundwaters sampled in Feb/Mar 29 (wells 222, 372, 4694, 472, 6722) had very unusual chemistry, with extremely low SiO 2, down to.5 mg/l, and low Fe, Mn, Ca, K, Mg, SO 4, Cu, As, Zn, and 222 Rn. All of these are old anoxic deep groundwaters with high methane, indicating that peat layers from swamp conditions are involved and that the unusual hydrochemistry is a result of a secondary stripping process in aquifer layers of peat. Other removal processes, such as biologic, ion exchange, or precipitation removal, are unlikely because most of the metals were removed too, and anions and cations were affected. Four of these wells were re-sampled in Feb 211 with a larger flow rate pump for better purging of these deep large-diameter wells. After stronger purging, more or less normal hydrochemistry was observed in these groundwaters. Figure 19 shows ph, HCO 3, SiO 2, and F versus MRT. The unusual chemistry results from insufficient purging of the wells are added with black circles, connected by a dashed line to the values after sufficient purging. These results demonstrate how dramatically hydrochemistry in the Ruataniwha groundwater environment can change if the water has sufficient contact time with the organic matter. ph shows a good correlation with MRT in the Ruataniwha groundwaters. Young waters have a ph of around 7, increasing to about 8 for waters with MRT 2 years. A similar relationship was found in other hydrogeological systems of New Zealand (Morgenstern & Daughney, 212). The insufficiently purged wells had extremely high ph, up to nearly 1 for well 472 (#3). Well 472 has extreme hydrochemistry for many parameters, including extremely high CH 4, which indicates peat layers as the cause. Bicarbonate overall also shows a good correlation to MRT, apart from two outliers (samples #3 and #24). Well 472 had a bicarbonate concentration reflecting insufficient purging, in line with the trend of the other samples, but increased in bicarbonate concentration after sufficient purging (> 3 well volumes). SiO 2 also shows the general trend of increasing concentration with time. Note that the result for sample #2 is after insufficient purging, as this well was not re-sampled. However the data indicated by the ellipse in Figure 19 follow a different trend. All of these samples are from the south eastern part of the Ruataniwha Basin, indicating the presence of an aquifer material with lower silicate solubility. Fluorite concentration in groundwater shows an excellent correlation to MRT (apart from the very unusual sample #3), indicating dissolution of fluorite-bearing minerals is homogenous throughout the basin. GNS Science Report 212/23 35

42 1 oxic anoxic unknown insufficient purging 4 3 ph (lab) Bicarbonate (mg/l) MRT [y] 1 2 MRT [y] SiO2 (mg/l) south-eastern part of basin F (mg/l) MRT [y] MRT [y] Figure 19 ph, HCO 3, SiO2, and F versus MRT in years. Small black open circles indicate wells with insufficient purging. In summary, the hydrochemistry data of most of the groundwaters in the Ruataniwha Basin follow similar trends over time. This indicates similar hydrochemistry processes throughout the Ruataniwha Basin. Nutrients NO 3 and PO 4 are shown in Figure 2. Only young groundwaters recharged after the start of industrial agriculture shortly after World War II (Morgenstern & Daughney, 212) show elevated nitrate concentration above 1 mg/l NO 3 -N (indicated by the box). In contrast, only the old groundwaters show elevated PO 4 concentration no data fall into the dashed line box. This indicates that the source of PO 4 in groundwater is purely from natural leaching processes from the aquifer material, and that the fertiliser phosphate is still held in the soil and has not yet broken into the groundwater. GNS Science Report 212/23 36

43 8 oxic anoxic unknown NO3-N (mg/l) 4 28 PO4 (mg/l) MRT [y] MRT [y] Figure 2 NO 3 and PO 4 versus MRT. 1. RECHARGE SOURCE AND FLOW DYNAMICS The Ruataniwha Plains groundwater system, as it has evolved during sea level changes, tectonic activity, and geomorphic processes, has a complicated hydrogeological setting with multi-layered aquifer systems in gravel, sandstone, pumice and limestone strata. This heterogeneous aquifer structure is reflected in the hydrochemistry, gas, and age tracer data, which show high variability. The high variability in hydrochemistry and alternating hydrochemistry over time in several deep wells indicates that pockets of water with different hydrochemistry occur, and can coexist in close proximity to each other. Wells with alternating hydrochemistry may intersect the boundary between such pockets and, depending on seasonal changes in recharge and groundwater abstraction, the different reservoirs are activated alternatingly. The recharge source, as indicated by hierarchical cluster analysis, stable isotopes and Cl, and dissolved gases and Cl is summarised in Table 7. For 21 locations it was possible to obtain an indication of the recharge source. These indicators are not always consistent for any one sample location, which is not unexpected in such a complex aquifer system (in terms of lithology and hydraulic parameters). Of the groundwater samples with tracer signatures indicating rain or river recharge, seven of them show signs of river recharge, indicating a strong interaction between river and groundwater in the Ruataniwha Basin. GNS Science Report 212/23 37

44 Table 7 Indication of recharge source as derived from the various methods. Grey background indicates samples with insufficient purging. Bore Bore No. # Depth Recharge source derived from m HCA SI-Cl gases-cl rain riv rain riv riv rain rain rain riv rain rain rain rain rain rain rain riv riv rain rain riv riv rain rain rain rain riv riv rain rain riv riv riv rain rain rain rain rain WaipR 8 riv riv riv TukitR 9 riv riv riv TukitR 11 riv riv riv DC Spring 14 rain riv riv L Spring 15 rain riv *Riv = River. The groundwater wells with overall indication for river recharge are: 143, 1518, 243, 378, 372, and 4694, and the Drainage channel spring. The wells with overall indication for local rain recharge are: 1381, 1452, 1558, 1655, 1944, 2579, 314, 3426, 3852, 6719, 1942, and 115. The remaining sites have no or an inconsistent indication of recharge source. The spatial distribution of indication of recharge source is shown in Figure 21. Only groundwaters in the vicinity of the large rivers (Waipawa and Tukituki) show a signature of a river recharge source. This may indicate that only these large rivers had sufficient energy to transport significant quantities of debris to form alluvial gravel deposits significant enough to allow good hydraulic connection of the deep groundwater flow system to the present river bed, with a significant fraction of river-recharged water in the deep groundwater. River-recharged groundwater is observed only in the lower reaches of these rivers, downstream from losing stretches of the rivers. Oxic groundwater is present only in the vicinity of Waipawa River, indicating that only this river has deposited relatively clean gravel aquifers without organic matter that would otherwise deplete the oxygen. A very distinct river GNS Science Report 212/23 38

45 recharge signature is observed in the groundwater of well This well is located close to where the Waipawa River exits the basin, in the gaining stretch of the rivers. This indicates that the groundwater in this area is river water that was lost from the river further upstream, and is upwelling back to the surface at the end of the basin. The springs in this area (Limestone and Drainage channel) show a mixed signature between rain and river recharge, indicating that this is the re-surfacing shallow groundwater, lost from the river upstream and mixed with rain-recharged groundwater from the surface. All the groundwaters in the southern part of the basin in the vicinity of small rivers and streams show a pure rain recharge signature, indicating an absence of gravel deposits significant enough to connect the river/streams to the deep groundwater system in this area. Figure 21 Spatial distribution of the indication of recharge sources. Blue triangles indicate river recharge, green triangles indicate recharge from local rain. Of the three triangles for each site, each represents one method for determining recharge source: left hierarchical cluster analysis; top stable isotopes and Cl; right Ar-N 2 -Cl. GNS Science Report 212/23 39