Assessment of Nitrogen and E. coli Groundwater Quality in the Hawke s Bay Region, May 2010 EMT 10/20 HBRC Plan No 4204

Transcription

1 Assessment of Nitrogen and E. coli Groundwater Quality in the Hawke s Bay Region, 2008 May 2010 EMT 10/20 HBRC Plan No 4204

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3 Environmental Management Group Technical Report Environmental Science Section Assessment of Nitrogen and E. coli Groundwater Quality in the Hawke s Bay Region, 2008 Prepared by: Matt Dodson Reviewed by: Dougall Gordon Graham Sevicke-Jones Approved: Darryl Lew, Group Manager Environmental Management May 2010 EMT 10/20 HBRC Plan Number 4204 Copyright: Hawke s Bay Regional Council

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6 EXECUTIVE SUMMARY The Nitrate sampling programme was initiated in 2003 in response to research that showed that some forms of Nitrogen and faecal contamination can potentially have adverse health effect on humans and stock. The Nitrate sampling programme currently samples at a five yearly interval. This report summaries the result and findings of the second sampling round conducted in The survey well selection was conducted in 2003, with 145 wells (<30m deep) being sampled. The 2008 survey attempted to re-sample all these wells. Of the 145 only 115 wells were re-sampled because of problems in accessing sites and changes to the well head or pump configurations (a 21% reduction in sites in five years). Generally Hawke s Bay groundwater has low concentrations of Nitrogen species (Ammoniacal, Nitrate and Nitrite) and low counts of Escherichia coli (E. coli). However, in unconfined aquifers Nitrate Nitrogen concentrations commonly exceed 5.65 mg/l suggesting they are vulnerable to surface contamination. The most common forms of Nitrogen in Hawke s Bay groundwater are Ammoniacal Nitrogen and Nitrate Nitrogen. Ammoniacal Nitrogen generally occurs in anaerobic, confined groundwater and at various depths. Nitrate Nitrogen typically occurs in aerobic and more often than not in unconfined groundwater. Nitrate Nitrogen concentrations appears to be restricted to shallow depths (<25m) however this may be an artefact of the programmes well selection methodology. Most sites in this Nitrate sampling programme have only two points which is not enough to determine a reliable trend. Therefore an attempt was made to define temporal trends using the survey data and data from the State Of Environment (SOE) groundwater quality programme. The SOE wells have been deliberately sited at various depths and locations. Here they are used because they have been sampled since approximately 1991 and they have up to 62 points per determinant. Seven SOE wells showed an increases trend for Nitrate Nitrogen and five SOE wells show a trend (both increasing and decreasing) for Ammoniacal Nitrogen (SOE wells n=41). Recommended changes to the well selection methodology and sampling interval are made. Alter the selection method to allow more wells to be sampled in area that have been identified as being susceptible to surface contamination. Review the frequency in which the survey is conducted. Page i

7 TABLE OF CONTENTS EXECUTIVE SUMMARY... I TABLE OF CONTENTS... II LIST OF FIGURES... III LIST OF TABLES... III 1.0 INTRODUCTION BACKGROUND Aims and Objectives Nitrogen and E. coli Land use Drinking-water standards SAMPLING AND ANALYTICAL METHODS Well selection Sampling procedures Analytical methods SURVEY RESULTS AND DISCUSSION survey results Data trends Depth trends Spatial trends Wairoa District Hastings Napier District Central Hawke s Bay District Temporal trends Survey data SOE data SUMMARY, CONCLUSIONS AND RECOMMENDATIONS Summary and Conclusions Recommendations REFERENCES GLOSSARY APPENDIX A APPENDIX B APPENDIX C Page ii

8 LIST OF FIGURES Figure 1 Map showing the Nitrate Nitrogen results for the 2008 survey Figure 2 Map showing the Ammoniacal Nitrogen results for the 2008 survey Figure 3 Map showing the E. coli results for the 2008 survey Figure 4 Scatter plot of Total Nitrogen against Nitrate Nitrogen Figure 5 Scatter plot showing Total Nitrogen against Ammoniacal Nitrogen Figure 6 Scatter plot showing Ammoniacal Nitrogen against Nitrate Nitrogen Figure 7 Scatter plot showing average Nitrate Nitrogen per well against the depth of that well Figure 8 Scatter plot showing the average Ammoniacal Nitrogen per well against the depth of that well LIST OF TABLES Table 1 Percentage of land uses of the Heretaunga and Ruataniwha Plains Table 2 Current discharge to land consents in the Hawke s Bay Region classified by activity Table 3 List of determinants, methods, detection limits, and laboratories used in the 2003 and 2008 surveys Table 4 Key statistics from the 2008 survey by determinant Table 5 Key statistics from the 2008 survey by determinant for the Ruataniwha and Heretaunga Plains Table 6 Results from the 2008 survey Table 7 Table showing key statistics and the results from the Kruskal Wallis test for the SOE wells Table 8 Results of the Mann Kendall test conducted in TimeTrends Table 9 Results of the Seasonal Kendall test conducted in TimeTrends Page iii

9 1.0 INTRODUCTION This survey was initiated in 2003 by the Hawke s Bay Regional Council to assess the concentrations of Nitrogen species (Ammoniacal, Nitrate and Nitrite) and faecal contamination in groundwater in the Hawke s Bay region. Faecal contamination in groundwater is indicated by the presence of faecal coliforms (analysed for the 2003 survey) and Escherichia coli (E. coli) (analysed for in the 2008 survey). The impetus for this survey was research findings that some forms of Nitrogen and faecal contamination can potentially have adverse health effects on humans and stock. Close et al (2001) and Larking and Baalousha (2007) have summarised the current knowledge of the health effects of Nitrate on humans. Currently sampling and analyses for this survey is conducted every five years. This report summarises the results and finding of the second sampling round conducted in August BACKGROUND The intention of this section is to state the aims and objectives of the study, to briefly review the current thinking on Nitrogen and E. coli contaminant sources, to present some qualitative land use information and to introduce the relevant limits set by the New Zealand drinking-water standards. 2.1 Aims and Objectives The purpose of this report is to: Gauge the concentrations of Nitrogen species (Ammoniacal, Nitrate and Nitrite) and E. coli counts in groundwater within the Hawke s Bay region. To attempt to identify any spatial trends and any temporal trends within the Nitrogen species dataset. To provide suggestions to improve the survey design and implementation of this sampling programme. 2.2 Nitrogen and E. coli Nitrogen species (Ammoniacal 1, Nitrate and Nitrite) occur naturally in groundwater, often at low concentrations. Higher concentrations of these Nitrogen species are often considered to be sourced from human induced activities such as fertilizer application, landfill leachate, effluent and wastewater discharges, amongst others (Close et al, 2001). Under these conditions, the Nitrogen is applied at or near the ground surface, with a proportion being fixed in the soil or being consumed by biologically entities. Excess Nitrogen can travel down to the water table and into the groundwater system. Nitrogen in groundwater can take the following forms. Total Nitrogen Organic Nitrogen Ammonia Nitrogen Nitrate Nitrite Total Kjeldahl Nitrogen (TKN) Organic Nitrogen Ammonia Nitrogen Total Nitrogen TKN Nitrate Nitrite E. coli is a bacterium that lives in the gut of warm blooded animals but can live outside the body for relatively short periods (usually days). Its presence in groundwater indicates faecal contamination. Typically, E. coli in groundwater is sourced from septic tanks or other effluent and wastewater discharges. Under these conditions E. coli migrates from the surface or near surface into groundwater. 1 To convert Ammoniacal Nitrogen to Ammiona Nitrogen, multiple the Ammoniacal Nitrogen value by Page 1

10 2.3 Land use The Hawke s Bay Regional Council conducts a five yearly land use survey for the Heretaunga and Ruataniwha Plains. The survey is essentially a snapshot of the land use taken at a distinct time period. The survey does not take into account temporal changes e.g. crop rotation, commodity prices (per comms. J Phillips 2009). The results from the 2008 survey are listed in Table 1. From Table 1 we can see that the major use of land in the surveyed area is pasture, with significant amounts of cropping, orchards and vineyards. No land use data is available for the remainder of the region. Table 1 Percentage of land uses as determined from the 2008 land use survey of the Heretaunga and Ruataniwha Plains. Class Percentage Class Percentage Orchards 4.3% Bareground 0.4% (=cultivated) Citrus 0.0% Plant nursery 0.1% Nuts 0.0% Cemetery 0.0% Berries-vines 3.8% School 0.0% Crops 5.1% Urban 2.0% Grains 5.3% Industrial 0.5% Pasture 70.8% Quarry 0.1% Forest 5.1% River/water 1.5% Wetland vegetation 0.2% Transport route 0.1% Recreational/ sports ground 0.7% Tabulated below are the numbers and percentages of current discharge to land consents by activity issued in the Hawke s Bay region (Table 2). Table 2 shows that the almost half of the discharge to land consents are issued for residential purposes. Table 2 Current discharge to land consents in the Hawke s Bay Region classified by activity. Activity Description Sum % Animal waste and/or effluent Dairy, piggery and poultry etc % Plant waste and/or effluent Vineyards, wineries and cropping etc % Commercial or industrial waste and/or effluent Processing plants, industrials, manufacturing etc % Municipal waste and/or effluent Public services etc % Landfill waste/or effluent Landfills and hardfills etc % Quarry waste and/or effluent Quarries etc % Residential waste and/or effluent Septic tanks etc % Other Napier Port, Marine farms etc % Total % 2.4 Drinking-water standards The New Zealand drinking-water standards (DWSNZ, 2005) have defined maximum acceptable values (MAV) for Nitrate Nitrogen, Nitrite Nitrogen and E. coli, as follows; Page 2

11 Nitrate Nitrogen 11.3 mg/l (half MAV mg/l). Nitrite Nitrogen 0.2 mg/l. E. coli <1 cfu 3 in 100ml of sample 3.0 SAMPLING AND ANALYTICAL METHODS The purpose of this section is to record the pertinent sampling and analytical methodologies for future reference. 3.1 Well selection The initial well selection was conducted in 2003 and it is described fully in Larking and Baalousha (2007). In brief, a grid 7 x 7km grid was superimposed over the region and up to nine shallow wells (<30m depth) per grid were chosen 4. The selected wells were visited and quality assured. These wells were then assessed using a numerical rating system resulting in 192 wells being deemed suitable. However only 145 sites were actually sampled due to errors associated with the quality assurance. For the 2008 survey the intention was to re-sample all of the 145 sites sampled in However, only 115 wells were re-sampled (a 21% reduction in sites). In most cases the wells were not resampled because suitable sampling points could not be located or there was no access to the well. 3.2 Sampling procedures The National Groundwater Monitoring Programme sampling protocol (MfE, 2006) was used to collect samples in the 2008 survey. The sampling protocol involves six steps; 1. Initial preparation; calibrating field meters, gathering sampling equipment etc. 2. At first arriving at a site; confirming it is the correct well, that there is a suitable sampling point and checking the pump set up to confirm the source of the water to the sampling point (i.e. direct off well, pressure tank etc). 3. Purging the well; this involves measuring the depth to the water table where appropriate, calculating the volume of water to be purged (which is at least three times the volume of the casing), installing a pump if required, turning on the pump, setting up the field meters (preferably in the flow cell) and then purging the well until the three volumes of the casing has been removed and the field meters are recording steady values. 4. Collecting the samples; sterilising the sample point with alcohol and then collecting the E. coli sample and collecting a field filtered sample in an unpreserved bottle. 5. Site clean-up. 6. Sample transport and delivery; samples stored in chilled bins and either delivered directly to the laboratory (for E. coli) or overnight couriered to the laboratory (remaining determinants). 3.3 Analytical methods Listed in Table 3 are what determinants were analysed for in the surveys, the laboratory used, the analytical method used and the detection limit. 2 Half MAV is used in the DWSNZ (2005) as a threshold for Nitrate. 3 cfu = colony-forming unit. 4 There is only one well deeper than 25 mbgl. Page 3

12 Table 3 List of determinants, methods, detection limits, and laboratories used in the 2003 and 2008 surveys. Determinant Laboratory Method Detection limit Year Total Ammoniacal Nitrogen (Total NH 3 -N) Nitrate Nitrogen (NO 3 -N) Nitrite Nitrogen (NO 2 -N) Nitrate Nitrogen + Nitrite Nitrogen (NNN) Hills (Hamilton) Hills (Hamilton) Hills (Hamilton) Hills (Hamilton) Phenol/hypochlorite colourimetry. Calculation: (Nitrate N + Nitrite) Nitrite N Automated Azo dye colourimetry, Total oxidised nitrogen. Automated Azo dye colourimetry, Total oxidised nitrogen. Automated cadmium reduction Faecal coliforms Kersons (Napier) Membrane Filtration E. coli NZ Laboratory Membrane Service Ltd Filtration (Hastings) 0.01mg/l 2003/ mg/l 2003/ mg/l 2003/ mg/l mg/l cfu s/100mls cfu s/100mls 2008 The only major change between the two surveys was the decision to change analysing for faecal coliforms to E. coli. E. coli is considered to be a more robust indicator of faecal contamination because it only lives in the gut of warm blooded animals and can be detected with greater analytical certainly. 4.0 SURVEY RESULTS AND DISCUSSION The results from the 2008 survey are presented in this section. Leading on from the results, the analysis and interpretations will be discussed. Here the data has been explored to see which Nitrogen species are most common in Hawke s Bay groundwater, to determine trends with depth, geographical trends and attempt to define trends with time survey results A subset of the results of the 2008 survey is shown graphically in Figure 1 (Nitrate Nitrogen), Figure 2 (Ammoniacal Nitrogen) and Figure 3 (E. coli). A table of all the data is provided in Appendix A. Shown are some key statistics determined from the 2008 survey results (Table 4). Shown in Table 5 are key statistics from the Heretaunga and Ruataniwha Plains. These tables identify the number of sites that exceeded the MAV per determinant. Generally, the 2008 results indicate that the water in most of the sampled wells meets the DWSNZ (2005) standards. Furthermore, except for the proportion of wells exceeding the Nitrate Nitrogen half MAV, the total number of wells and proportions of exceedances for the measured determinants decreased from 2003 to Page 4

13 Table 4 Key statistics from the 2008 survey by determinant. See section 2.4 for MAV and Table 3 for detection limits and a key to the abbreviations. N = 115. E. coli cfu/ 100ml NNN mg/l NH 3 -N mg/l NO 2 -N mg/l NO 3 -N mg/l Average Median Std Dev Min 0 <0.002 <0.01 <0.002 <0.002 Max >MAV >0.5 MAV 12 Table 5 Key statistics from the 2008 survey by determinant for the Ruataniwha and Heretaunga Plains. See Figure 1 for locations, section 2.4 for MAV and Table 3 for detection limits and a key to the abbreviations. Heretaunga n = 55, Ruataniwha n = 14. E. coli cfu/ 100ml NNN mg/l HERETAUNGA NH 3 -N mg/l NO 2 -N mg/l NO 3 -N mg/l Average Median Std Dev Min 0 <0.002 <0.01 <0.002 <0.002 Max >MAV >0.5 MAV 5 RUATANIWHA Average Median Std Dev Min 0 <0.002 <0.01 <0.002 <0.002 Max >MAV >0.5 MAV 4 Page 5

14 Figure 1 Map showing the Nitrate Nitrogen results for the 2008 survey. Note that two wells in the Heretaunga Plains with Nitrate Nitrogen values between mg/l are obscured behind the red dots in the image. Page 6

15 Figure 2 Map showing the Ammoniacal Nitrogen results for the 2008 survey. Page 7

16 Figure 3 Map showing the E. coli results for the 2008 survey. Page 8

17 NO 3 -N (mg/l) 4.2 Data trends In order to gain a better understanding of the form Nitrogen occurs in groundwater within the region all the data from all the Nitrate survey sites and data from the State Of Environment (SOE) groundwater quality programme was extracted from PUDDLE 5. The SOE groundwater quality dataset was added to increase the total size of the population and to provide some data from deeper bores. This dataset revealed that Nitrite Nitrogen occurs in low concentrations (highest concentration 0.5 mg/l, n = 1164). When plotting Nitrate Nitrogen against Nitrate Nitrogen + Nitrite Nitrogen (plot not shown) a near 1:1 linear trend is found (y = x, r 2 = 0.99). This trend indicates that virtually all of the Nitrate Nitrogen + Nitrite Nitrogen occurs as Nitrate Nitrogen. It was also found that Ammoniacal Nitrogen constitutes 57% (median value of n = 560) of Total Kjeldahl Nitrogen. This suggests that Nitrogen occurs mainly as Nitrate Nitrogen and Ammoniacal Nitrogen in Hawke s Bay groundwater. Similar conclusions were drawn by White and Daughney (2004) for the Ruataniwha Plains. Shown in Figure 4 and Figure 5 is Total Nitrogen plotted against Nitrate Nitrogen and Ammoniacal Nitrate. Both scatter plots show a V shaped trend. This pattern indicates two populations in the data. The first population plots on a near 1:1 linear trend line. This pattern indicates that most of Total Nitrogen occurs as the other plotted determinant. The second population plots along the x axis. This pattern indicates that the other determinant is effectively absent. These figures further support the conclusion that Ammoniacal Nitrogen and Nitrate Nitrogen are the most common form of Nitrogen in Hawke s Bay groundwater. 16 Total Nitrogen vs. Nitrate Nitrogen TN (mg/l) Figure 4 Scatter plot of Total Nitrogen against Nitrate Nitrogen. 5 PUDDLE is the Hawke s Bay Regional Councils water quality database. Page 9

18 NO 3 -N (mg/l) NH 3 -N (mg/l) 16 Total Nitrogen vs. Ammoniacal Nitrogen TN (mg/l) Figure 5 Scatter plot showing Total Nitrogen against Ammoniacal Nitrogen. 16 Ammoniacal Nitrogen vs. Nitrate Nitrogen NH 3 (mg/l) Figure 6 Scatter plot showing Ammoniacal Nitrogen against Nitrate Nitrogen. Page 10

19 Depth (mbgl) Nitrate Nitrogen is plotted against Ammoniacal Nitrate in Figure 6. The pattern in Figure 6 shows that typically where Nitrogen concentrations are >~2.0 mg/l either Ammoniacal Nitrogen or Nitrate Nitrogen is present. A similar interpretation was presented by Rosen (2001) for the National Groundwater Monitoring Programme (NGMP) data. Nitrate Nitrogen occurs in aerobic waters and Ammoniacal Nitrogen occurs in anaerobic waters (Rosen and McNeill, 1996). 4.3 Depth trends Depth trends have been investigated by constructing scatter plots of average concentration per site against depth for all the sites in the Nitrate survey programme and the SOE programme (Figure 7 and Figure 8). Average concentrations have been used because some SOE sites have been sampled up to 62 times per determinant which when plotted obscures any apparent trends. 0 Average Nitrate Nitrogen vs. Depth Average NO 3 -N (mg/l) Nitrate sampling programme SOE 180 Figure 7 Scatter plot showing average Nitrate Nitrogen per well against the depth of that well. mbgl = meters beneath ground level. When viewing Figure 7 it should be remembered that the plotted points are predominately derived from the Nitrate Sampling programme which as part of the well selection methodology stipulated a maximum well depth of 30m. From Figure 7 we can see that shallow wells tend to have the highest average Nitrate Nitrogen concentrations. In fact apart from four sites, wells deeper than 25 mbgl have average Nitrate Nitrogen concentration of <1.5 mg/l. This pattern indicates that Nitrate Nitrogen contamination is most likely to occur in shallow groundwater (<25 mbgl) and suggests that the Nitrate is sourced from the ground surface. Similar conclusions were drawn by Rosen and McNeill (1996). Hereafter 1.5 mg/l is used as a threshold for surface contamination. The plot of depth against average Ammoniacal Nitrogen does not reveal any conclusive trends (Figure 8). The majority of the points in Figure 8 plot below 1.0 mg/l. There is considerable scatter in the data over 1.0 mg/l of Ammoniacal Nitrogen. Page 11

20 Depth (mbgl) 0 Average Ammoniacal Nitrogen vs. Depth Average NH 3 (mg/l) SOE Nitrate sampling programme Figure 8 Scatter plot showing the average Ammoniacal Nitrogen per well against the depth of that well. mbgl = meters beneath ground level. For the scatter plot, depth against E. coli (not shown), all individual data points were used (n=755). This is because E. coli is detected less frequently than Nitrate Nitrogen and therefore the graph is easier to decipher. When plotting E. coli counts against depth the trend is approximately similar to that shown in Figure 7. The main differences are that E. coli contamination is often reported down to depths of 60 mbgl and the numbers of samples with E. coli detected is less (n detections =44). Typically E. coli detections are encountered in shallow wells indicating surface contamination. E. coli detections at depth are likely to be caused by water leaking down the side casing or by sample contamination. 4.4 Spatial trends Spatial trends were investigated by viewing the 2008 survey data in ArcMap geographical information system (GIS). Additional GIS layers were used to assess the data, namely HB_aquifers (which displays the approximate boundary of the aquifer and aquifer description (unconfined or confined)), current consents layers, rivers and topography Wairoa District Nitrate Nitrogen concentrations are generally low (<1.5 mg/l) in the Wairoa District, except two shallow wells (<9m depth) in the Mahia alluvial aquifer. These wells (1325 and 2601) are located in sand and gravel deposits. Based on their depth, bore log, proximity to streams and Nitrate Nitrogen concentration, are likely to be unconfined and rainfall recharged. E. coli was not detected in any of the sampled wells. Page 12

21 Ammoniacal Nitrate concentrations are generally low (<0.3 mg/l 6 ), except two wells in Wairoa valley aquifer. These wells (1923 and 3498) had Ammoniacal Nitrogen concentration >3.6 mg/l is 17m deep and 3498 is 32m deep. There is no borelog or aquifer test information stored on WellStor 7 about these wells. However the Wairoa valley aquifer is considered to be confined based on the presence of flowing artesian wells and bore logs (Cameron, 1999). The low Nitrate Nitrogen and high Ammoniacal Nitrogen concentration suggests that this confined aquifer is a closed system in respect to oxygen Hastings Napier District Nitrate Nitrogen concentration in the Hastings Napier District are generally low (<1.5 mg/l), but there are clusters of higher concentrations (>1.5 mg/l) in areas. The most notable area is in the Western, unconfined, part of the Heretaunga aquifer (Dravid and Brown, 1997), and particularly South of Roy s Hill. In the unconfined part of the aquifer 10 out of the 22 wells surveyed in 2008 have Nitrate Nitrogen concentrations >1.5 mg/l (45%), and 23% of these wells have Nitrate Nitrogen concentrations >half MAV. This indicates that the Heretaunga unconfined aquifer is particularly vulnerable to contamination. It also suggests that these wells are recharged significantly by rainfall. At the Ngaruroro River Nitrate Nitrogen concentrations are typically low (Site 298 median NO 3 -N mg/l n=51, Site 299 median NO 3 -N 0.03 mg/ n=71, Site 353 median NO 3 -N mg/ n=75 and Site 354 median NO 3 -N mg/ n=72). Three shallow wells (<18m deep) located in the Eastern part of the Heretaunga aquifer have Nitrate Nitrogen concentrations over 1.5 mg/l but less than 5.65 mg/l. The bore logs of these wells (669, 4174 and 10496) indicate gravels and fines, suggesting an unconfined aquifer overlying the major confined aquifers. Within the Hastings - Napier District three wells had detections for E. coli is an 8m deep well, within gravels, located in the upper Esk valley is a 10m deep bore, in fluvial deposits, located in the Heretaunga unconfined aquifer is 20m deep, in intebedded fines and alluvium, in the Poukawa Basin aquifer. The depth and geological logs suggests that these wells are connected to the land surface. Ammoniacal Nitrogen concentrations in the Hastings - Napier District are generally low (<0.3 mg/l) however two wells in this region exceeded 1.5 mg/l and a further nine wells had concentrations between mg/l (15m deep) and 3775 (16.5m deep) had Ammoniacal Nitrogen concentrations >2.8 mg/l is located in Bayview and 3775 is near the Western boundary of the Heretaunga aquifer. The bore logs in both wells show >5m thick layers of fines suggesting some degree of confinement. For the nine wells with Ammoniacal Nitrogen concentrations between mg/l, all are less than 23m deep and all have at least >3m of fines layer/s recorded in there bore log. This suggests some degree of confinement. Six of these wells are located in the Heretaunga confined aquifer Central Hawke s Bay District 50% of the surveyed wells in the Central Hawke s Bay District (total n=24) had Nitrate Nitrogen concentration >1.5 mg/l, 20% >half MAV and 10% >MAV. The maximum depth of these wells is 23m but typically bore depths were less than <14m. Bore logs of these wells show a variety of lithofacies, including fine and course grained fluvial deposits, alluvial deposits and coastal sand deposits. In the Ruataniwha basin many of the wells with Nitrate Nitrogen concentration >1.5 mg/l occur in the unconfined part of the basin. Three wells in the Central Hawke s Bay District had detections for E. coli.1602 (6.1m deep) located in the sand deposits near Porangahu, 2860 (17m deep) located in siltstones in the Papanui Stream mg/l was chosen as a threshold by using the natural breaks classification in GIS. 7 WellStor is the Hawke s Bay Regional Councils wells database. Page 13

22 Valley aquifer and 2370 (14m deep) located on the intermediate terraces of the Ruataniwha Basin. The depth of these wells suggest that they are connected to the land surface. Only one well in the Central Hawke s Bay had an Ammoniacal Nitrogen concentration above 0.3 mg/l. For the 2008 survey 1944 (20.4m deep) had an Ammoniacal Nitrogen concentration of 0.64 mg/l. The bore log for this well revealed that the screen was located between fine grained layers suggesting some degree of confinement. 4.5 Temporal trends Two separate datasets have been explored in attempt to define temporal trends. The first dataset is the results from the two Nitrate surveys (Appendix A). The bulk of this dataset has a maximum of two sets of results per site and as such only the most basic assessment is possible. The second dataset used is the SOE groundwater quality sites. These wells have up to 62 results per determinant, some with up to 15 years of data. However, the SOE wells have been chosen to represent different geographical locations and depths. As such many of these wells are unlikely to show Nitrate Nitrogen or E. coli contamination Survey data The Ammoniacal Nitrogen and Nitrate Nitrogen survey data was analysed by subtracting the 2003 result from the 2008 (Appendix A). An error of twice the detection limit was used to take into account analytical errors. The Nitrate Nitrogen data was plotted in GIS using a manual classification of >0.004 mg/l, < to > mg/l (taken to be no major change) and < mg/l. The Ammoniacal Nitrate data was plotted as >0.02 mg/l, <0.019 to > mg/l (taken to be no major change) and <-0.02 mg/l. This method was not applied to the faecal contamination indicators because of the change from analysing for faecal coliforms to E. coli. For Nitrate Nitrogen data in the Wairoa District, the classification showed that there was a reduction in Nitrate Nitrogen concentrations in seven wells (max decrease -8.6 mg/l, median mg/l). Four wells showed an increase in Nitrate Nitrogen concentrations (max increase mg/l, median 0.76 mg/l). In the Hastings - Napier District 40 bores showed an increase in Nitrate Nitrogen concentration (max increase mg/l, median 0.16 mg/l), 25 showed a decline (max decrease -6.5 mg/l, median mg/l) and the remainder (n=15) show no major change. In the Central Hawke s Bay District 15 wells showed an increase (max increase mg/l, median 0.38 mg/l) and four showed an decreased in Nitrate Nitrogen concentrations (max decrease mg/l, median mg/l). For Ammoniacal Nitrogen data in the Wairoa District a single well recorded an increase (+0.28 mg/l), with three wells recording a decline (max decrease mg/l). In the Hasting Napier District 12 wells each recorded an increase (max increase +2.8 mg/l, median mg/l) and 18 wells recorded a decrease in Ammoniacal Nitrogen concentrations (max decrease mg/l, median mg/l). In the Central Hawke s Bay District six wells showed a decrease (max decrease mg/l, median -0.05) and four wells showing an increase (max increase +1 mg/l, median mg/l). The method employed here is simple because many of the wells do not have the data to conduct more robust analysis. Furthermore, it must be acknowledged that two points do not make a reliable trend and that five years is a very long interval between samples. However, very broadly speaking the results fit with the conceptual model of the author. In that, the results suggest that the unconfined Heretaunga aquifer and the Ruataniwha Basin are the most responsive to changes in Nitrate Nitrogen concentrations. The data also suggest that Ammoniacal Nitrogen concentrations are generally stable throughout the region. Page 14

23 4.5.2 SOE data Nitrate Nitrogen and Ammoniacal Nitrogen data for the SOE wells (n=41) was examined in TimeTrends 8, statistixl and excel. E. coli was not examined because it was only found in 4% of the SOE analyses (n=622) and no well had consecutive detections. Initially the SOE well data was tested for normality by calculating the kurtosis and skewness for each data set. Only one determinant came close to be normal distributed, being Ammoniacal Nitrogen for well (Appendix B). The data was then tested for seasonality by using the Kruskal Wallis test in statistixl. Seven wells had seasonal patterns for Nitrate Nitrogen and one for Ammoniacal Nitrogen (P <0.05) (Appendix B). As the data is effectively nonparametrically distributed those without seasonality where analysed with the Mann Kendall test to determine time trends.of the eight sites where seasonality was detected the seasonal Kandall test was employed. The trend tests revealed that seven wells exhibit an increasing trend for Nitrate Nitrogen and five wells exhibited both increasing and decreasing trends for Ammoniacal Nitrogen (Appendix C). Five wells were found to have either an increasing or decreasing trend for Ammoniacal Nitrogen. Two of these wells are located in the Mahia alluvium aquifer ( m deep, m deep). These wells have low median concentrations (<0.10 mg/l) and either a very slight decrease (1923) or increase (15017) in Ammoniacal Nitrogen concentrations over time. The remainder are located in the confined Heretaunga aquifer. 705 (41m deep) and 1940 (76m deep) have median concentration >0.3 mg/l but <0.5 mg/l. Both show an increasing concentration of Ammoniacal Nitrogen with time is 90m deep and one of the Awatoto piezometers. This well shows a relatively large decrease in Ammoniacal Nitrogen concentration with time (Appendix C). From the SOE well data set there are eight wells with median Ammoniacal Nitrogen concentrations 1.3 mg/l (47m deep) is located on the Mahia peninsula and is drilled into mudstone. There are three wells in the confined Wairoa valley aquifer ( m deep, m deep and m deep) spanning the length of the valley. There are three wells located in the confined Heretaunga aquifer (15002, m deep and m deep). The final wells is a free flowing artesian well in the upper Poukawa basin ( m deep). Generally it appears that Ammoniacal Nitrogen concentrations in the Hawke s Bay region are relatively stable. Five of the seven wells that show an increasing trend for Nitrate Nitrogen are located in the Hastings Napier District. Two of these wells are shallow (<30m) and likely to be unconfined (3301 and 10340). The other three wells are located in the confined Heretaunga aquifer at various depths ( m deep, m deep, m deep). These last three wells have low median Nitrate Nitrogen concentrations (<0.33 mg/l) and median annual Zen slopes between 0.00 and The low Zen slopes indicate that there is not a significant shift in concentration for these wells. The remaining two wells with an increasing Nitrate Nitrogen trend are located in the Ruataniwha Plains. Both wells are located in the Northern plains near State Highway is 56m deep free flowing artesian well and 2749 is 51m deep. Borelogs for both wells show gravel and clay layers which would indicate some degree of confinement. A five step drawdown test (with a single 8 TimeTrends is a NIWA program designed to carry out statistical tests Page 15

24 observation bore) has been conducted on 1518 which produced an S of , indicating a confined aquifer (average T of 180 m 2 /day). Listed in Appendix B are the median values for Nitrate Nitrogen for the SOE wells. It is interesting to note that seven of these wells have median Nitrate Nitrogen values >1.5 mg/l. Of these wells 2749 and 3301 have been discussed in the previous paragraphs (48m deep) and 3697 (89m deep) are piezometers at Tollemache, Hastings. All the piezometers at Tollemache are free flowing artesian. The deepest piezometer at Tollemache is 115m deep and has a median Nitrate Nitrogen concentration of 0.05 mg/l is 21m deep and located near Roy s Hill is 8m deep located just outside of Havelock North, and is likely to be unconfined. The final well 1558 is 23m deep well located in the Ruataniwha Plains near Ashcott. This data indicates that Nitrate Nitrogen contamination is occurring to greater depths than 30 mbgl and that its concentrations are increasing with time in parts of the Ruataniwha and Heretaunga aquifers. The Nitrate Nitrogen found in the confined wells (mentioned in the previous paragraphs) are considered to be sourced up gradient from there recharge areas. The high median Nitrate Nitrogen concentrations (>1.5 mg/l up to 5.95 mg/l) in the Heretaunga and Ruataniwha aquifers show that there is already significant localised surface contamination. 5.0 SUMMARY, CONCLUSIONS AND RECOMMENDATIONS 5.1 Summary and Conclusions The well selection for the survey is based on a grid pattern superimposed over the region and then selecting a subset of wells within that grid. In 2003, 145 wells were sampled. In 2008, 115 wells were sampled. This represents a 21% reduction in wells in a five year period. The results of the 2008 survey indicate that generally the groundwater in the Hawke s Bay region has low concentrations of Nitrogen species (Ammoniacal, Nitrate and Nitrite) and low counts of E. coli. Furthermore, most of the total numbers and proportions of exceedances per determinant decreased from 2003 to The most common forms of Nitrogen in Hawke s Bay groundwater are Ammoniacal Nitrogen and Nitrate Nitrogen. The data shows that either Nitrate Nitrogen or Ammoniacal Nitrogen is present where these determinants have a concentration of >2 mg/l. Nitrate Nitrogen commonly occurs in aerobic, unconfined groundwater systems and at shallow depths. However Nitrate Nitrogen also occurs in confined, deeper aquifers presumable with the Nitrogen sourced from their respective recharge areas. Ammoniacal Nitrogen mostly occurs in anaerobic, confined groundwater systems and at various depths. Temporal trends are difficult to determine in the survey data as there are too few data points to draw a robust conclusion. The SOE well data set (which has more data points) was used. While the SOE network has been designed to be representative of different locations and depths has provided invaluable information about surface contamination. Generally it seems as there is an increase in Nitrate Nitrogen concentrations in the Heretaunga and Ruataniwha basin. Ammoniacal Nitrogen appears to be stable. 5.2 Recommendations The survey well selection while objective does not target the most vulnerable parts of the aquifer. It is suggested that the wells sampled in the survey be predominately sited in the Page 16

25 unconfined part of the aquifer systems (Draivd and Brown, Brooks, Baalousha, 2009). Public supply wells (or wells upgradient of public supply wells) where possible should be included in the survey. The sampling frequency is too long. Two problems arise from the current sampling frequency; too many wells cannot be re-sampled (21% reduction since the last sampling round) and the length of time required before a statistical robust technique could be applied. It is suggested that sampling be conducted annually and the data analysed after a five year period. Sampling of spring fed surface water sites should be considered. Sampling for Total Nitrogen and Phosphorus species should be considered. The Hawke s Bay Regional Council have land use data collected at five year intervals. Analysis of land use data in conjunction with this survey would greatly improve the understanding of nutrients in the groundwater system. Page 17

26 6.0 REFERENCES Baalousha H. 2009: Ruataniwha Basin Modelling. A steady state groundwater flow model. Hawke s Bay Regional Council Internal technical report. Brooks T. 2006: Heretaunga Steady-state Ground-water Model. Hawke s Bay Regional Council Internal technical report. Cameron S. 1999: Northern Hawke s Coastal Groundwater Study. Hawke s Bay Regional Council Internal technical report. Close M.E; Rosen M.R and Smith V.R. 2001: Fate and transport of nitrates and pesticides in New Zealand aquifers. In Groundwaters of New Zealand, M.R Rosen and P.A. White (eds). New Zealand Hydrological Society Inc., Wellington. Pg Dravid P.N and Brown L.J. 1997: Heretaunga Plains Groundwater Study. Volume 1: Findings. Hawke s Bay. Geological and Nuclear Sciences client report 96/02. Lower Hutt, Geological and Nuclear Sciences. Larking R and Baalousha H. 2008: Nitrate in Groundwater in Hawke s Bay Region. Hawke s Bay Regional Council Internal technical report. Ministry for the Environment, 2006: A National Protocol for State of the Environment Groundwater Sampling in New Zealand. Wellington, Ministry for the Environment Ministry of Health, 2005: Drinking-water standards for New Zealand. Wellington, Ministry of Health. Rosen M.R. 2001: Hydrochemistry of New Zealand s aquifers. In Groundwaters of New Zealand, M.R Rosen and P.A. White (eds). New Zealand Hydrological Society Inc., Wellington. Pg Rosen M.R and McNeill. 1996: Nitrate concentrations and migration in groundwater beneath the Maraekakaho sheep feedlot, Hawke s Bay. Geological and Nuclear Sciences client report 96/02. Lower Hutt, Geological and Nuclear Sciences. White P.A and Daughney C: Regional groundwater quality and surface water quality model of the Ruataniwha Plains. Geological and Nuclear Sciences client report 2002/128. Taupo, Geological and Nuclear Sciences Page 18

27 7.0 GLOSSARY Ammoniacal nitrogen The sum of dissolved unionized ammonia gas (NH 3 ) and ammonium ion (NH 4 + ). Nitrate Nitrogen NO 3 -N. Nitrite Nitrogen NO 2 -N. Organic Nitrogen nitrogen compound that had its origin in living material. Total Kjeldahl Nitrogen The sum of organic Nitrogen and Ammonia Nitrogen. Page 19

28 Easting Northing FC 2003 ECOLI 2008 NH3-N (mg/l) 2003 NH3-N (mg/l) 2008 Δ from 2003 to 2008 NNN (mg/l) 2003 NNN (mg/l) 2008 Δ from 2003 to 2008 NO2-N (mg/l) 2003 NO2-N (mg/l) 2008 Δ from 2003 to 2008 NO3-N (mg/l) 2003 NO3-N (mg/l) 2008 Δ from 2003 to 2008 APPENDIX A Table 6 Results from the 2008 survey. Also shown are the 2003 results for wells sampled during both surveys and the difference between the two surveys ( = value). The red highlighted cells indicate values above their respective MAV limits and yellow highlighted cells indicate values above half the Nitrate Nitrogen MAV limit Page 20

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