IMPACT OF ANTHROPOGENIC ACTIVITIES ON GROUNDWATER QUALITY IN PUTTALAM DISTRICT

Transcription

1 IMPACT OF ANTHROPOGENIC ACTIVITIES ON GROUNDWATER QUALITY IN PUTTALAM DISTRICT K. M. S. M. KUMARASINGHE* and R. R. G. R. RAJAPAKSE Water Resources Board, 2A, Hector Kobbakaduwa Mawatha, Colombo 07 * ABSTRACT Chemical characterization of groundwater bodies was carried out at 144 locations in the Puttalam District of Northwestern Sri Lanka, with special focus on salinity as well as nitrate and phosphate contamination. Field measurements of temperature, ph, and electrical conductivity were made during sampling at monitoring points (dug wells/tube wells) representing both shallow and deep aquifers. The sampling was carried out during wet (February- March 2012) and dry (September-October 2011) seasons of the year. Chemical analyses of water samples were later made for Ca, Na, K, Mg, iron, chloride, sulphate, F, nitrate and phosphate. Study reveals that the groundwater quality in Puttalam and Vanathavillu areas is adversely affected due to high salinity in groundwater whereas Kalpitiya area is contaminated due to improper practices of pesticide and fertilizer applications. Nitrate contamination pattern in groundwater of some localized areas are shown as high as >10 mg/l whereas average value of more than 5 mg/l of shallow groundwater was reported in many villages of Kalpitiya area. Analytical results indicate high nitrate contamination in Puttalam urban area, which may be due to improper sanitation facilities. Relatively higher salinity levels were reported in the shallow aquifers associated with Mee Oya with no indication of increased salinity levels in the deep aquifers, implying salt-water intrusions at shallower levels. However, in some sampling points of the lagoonal as well as inland areas, site specific characteristics of increased electrical conductivity (EC) and salinity levels were observed. Elevated salinity and nitrate concentrations in shallow groundwater in intensive agricultural areas appear to be related to leaching of nitrates from soils due to over abstraction aggravating the condition resulting in sea water encroachment to the coastal stretches. The phosphate contamination in deeper aquifers could be identified in the upper part of Vanathavillu (Eluwankulama, Rahalmadu and Serakkuliya) indicating more than 2 mg/l of phosphate concentration. Key Words: groundwater pollution, Puttalam, fertilizer, resistivity, anthropogenic activities INTRODUCTION Groundwater plays a vital role in Puttalam area for drinking, agricultural and other domestic purposes. The surface water sources are limited in the area and not adequate to meet the everincreasing water demand. Paddy and other cash crops such as potatoes, onions, chillies and tobacco are extensively cultivated in the region. The use of surface water for drinking and other purposes is hindered due to poor quality. This aggravates the risk on groundwater sustainability and quality deterioration due to alarmingly higher abstractions levels. Groundwater contamination in terms of nitrate pollution in the area has been studied for several decades (Lawrence and Kuruppuarachchi, 1986; Mubarak et al., 1992; Kuruppuarachchi and Fernando, 1999; Liyanage et al., 2000; Jayasingha et al., 2011; Jayasingha et al., 2013). Present study deals with Puttalam, Vanathavillu and Kalpitiya District Secretariat Divisions, 129

2 Kumarasinghe, K.M.S.M. and Rajapakse, R.R.G.R. /Impact of anthropogenic activities on groundwater quality in Puttalam District which were identified as most affected areas of groundwater sustainability and quality deterioration. Puttalam and Vanathavillu District Secretariat Divisions are located between hydrological basins of Kala Oya and Mee Oya. This study is mainly to assess the groundwater chemistry in the region and identify the impact levels on groundwater due to excessive application of agrochemicals and pesticides. Further, the extent of salt-water intrusion due to over abstractions is also discussed. GEOLOGY OF PUTTALAM-KALPITIYA The coastal belt is underlain by unconsolidated Miocene sedimentary sequences whereas the inland territory of the study area underlain by rocks of Precambrian Wanni Complex (Cooray, 1984, Cooray, 1994). The spatio-temporal water chemistry of groundwater and flow behaviour changes drastically over different facies originated due to brittle block faulting, especially within the limestone formation (China Water Supply Project in Puttalam, 1987). Additionally, the land use pattern, geology, groundwater flow regime are the major governing factors in the variation of groundwater chemistry. Two distinguished temporal variations of water quality in groundwater were noted during wet and dry periods of the year. Depth-wise variations of groundwater chemistry were observed in limestone aquifer whereas hard rock aquifer sea formations had not indicated such variations in the area (Wetselaar et al., 1993). The occurrence of groundwater in the Miocene limestone formation confined to the North and Northwest coastal stretches of the island is characteristically different from the hard rock terrains of the interior of the country. Limestone formation is the prominent source for groundwater storage due to its extensively karstic nature and the presence of innumerable joints, fissures, solution cavities and chambers. Most of such openings are being constantly enlarged by solution due to the slightly acidic water which circulates in the underground drainage system using fissures and joints (Cooray, 1984). Shallow aquifers on coastal spits and bars are found in the Kalpitiya peninsula (Figure 1). These aquifers are re-charged mainly during the 3 to 4 months of rain of the wet maha season. The water in these aquifers then gets collected in the form of fresh water lenses floating above the denser saline water. The volume of fresh water in these aquifers usually expands during the rainy season and contracts during the dry periods with fluctuating brackish and saline boundaries. Any over-extraction from these fresh water lenses could result in the entry of underlying brackish water into fresh water. The aquifer in the Kalpitiya peninsular on a coastal lagoon 130 Figure 1. The aquifer in Kalpitiya Peninsula on a coastal spit

3 spit as shown in Figure 1 has a thinner lens of fresh water. It is more easily subjected to depletion or eutrophication. METHODS OF STUDY Literature Review The international guidelines of BS (British Standards) and ASTM (American Standards of Testing Methods) for methods of sample collection, analysis and selection of monitoring points were employed according to the issues identified. Information available on various issues related to groundwater contamination as a result of extensive agriculture, sea-water intrusion, over-exploitation and health implications were identified. Poor water quality reported in previous studies was also considered. Acquisition of required hydrogeological, land use, climatic, geological and previous water quality data and information of the region was also done. Preliminary Survey An extensive reconnaissance survey of all existing relevant data was carried out to understand the influence of regional and sitespecific groundwater flow regimes of the area. In the initial phase of the program, preliminary field inspections were carried out to identify the possible impacts on the groundwater due to various processes at site-specific level in the study area, i.e. waste disposal sites, potential pollution sites due to industrial effluents, popular agricultural regions, as well as poor water quality zones and high abstraction locations identified from the available data. Chemical Analysis During the present study, one hundred and forty four (144) water samples were analysed for electrical conductivity (EC), ph, total dissolved solids (TDS), phosphate and nitrate as in-situ tests to initially assess the geochemistry of groundwater. The monitoring points were established considering various issues (i.e. industrial pollution by effluents, high salinity resulting from over-abstraction, agricultural pollution, natural inherited formation characteristics etc.). Sampling was done in the shallow as well as deep aquifers. The sampling process was carried out during wet and dry seasons of the year considering the seasonal variation of water chemistry. Fifty two (52) water samples were collected during the dry season (September-October 2011). Based on the results of chemical analysis of these samples, additional 50 samples were collected during the wet season (February- March 2012). All samples were analysed for ph, EC, TH, TA, TDS, Ca, Na, K, Mg, Iron, Cl, Sulphate, F, salinity, nitrate and phosphate as well as heavy metals (Mn, Cu, Pb and Cd). Bacteriological testing was done at identified vulnerable locations. Assessment of Flow Regime The groundwater head measurement was monitored for identified groundwater sources to assess the flow behaviour of the area. The elevation of the monitoring point was determined by Differential GPS with respect to mean sea level (MSL). The flow pattern was observed on a quarterly basis to identify the spatio-temporal variation while monitoring groundwater level fluctuations. Two-dimensional Resistivity Survey In addition to water quality and head monitoring, the 2-dimensional (2D) imaging resistivity surveys were carried out. The aim of these surveys was to interpret the subsurface hydrogeological, geological and structural conditions in the limestone aquifer. These included structurally weak zones, thickness of soil overburden, weathered rock and possible changes in groundwater quality with depth. The AGISuperSting R8/IP is a 8-channel memory earth Terrameter with higher accuracy and lowest noise levels were used for resistivity survey. The system includes SuperSting R8/IP equipment, switching unit which is capable of 131

4 Kumarasinghe, K.M.S.M. and Rajapakse, R.R.G.R. /Impact of anthropogenic activities on groundwater quality in Puttalam District handling 112 electrodes and a passive cable system of 5 m spacing to connect the 112 electrodes. Therefore, one survey line is expanded to 560 m on the ground surface and the probing depth is approximately m below the ground level depending on the resistivity array chosen. Ten (10) numbers of such surveys were carried out at locations where subsurface hydrogeology is required for interpretation, thus enabling subsequent assessment of regional hydrogeology of the entire area. The surveys were carried out at 5 m electrode spacing at maximum and 2 m in certain places depending on the requirement of probing depth. Dipole-Dipole technique was applied in most situations considering the necessity of interpreting the subsurface depth beyond 100 m in the limestone terrain. The raw data were analysed in the EarthImager 2D software to obtain the inversion images where the subsurface resistivity as a 2D section with a span of 560 m maximal is indicated. The careful and accurate interpretation of these sections was done through the personal experience with the support of additional background information. Borehole data, quality data, geological and structural information play a major role in the final assessment. RESULTS AND DISCUSSIONS Results of the nitrate and phosphate analyses are contoured (see Figures 2 and 3). Nitrate levels are particularly higher than the permissible level (10 mg/l) towards the Kalpitiya Peninsula. Eththale, Alankuda, Norachchoelai, Minniya and Nirmalapura villages (more than 70% of the monitoring locations of these villages) are in Kalpitiya DSD which is mostly affected by nitrate contamination of shallow groundwater. However, there is a considerable threat of nitrate contamination throughout the Kalpitiya area (nitrate level exceeds 5 mg/l in many villages of Kalpitiya area). This elevated nitrate level in groundwater is possibly due to the extensive application of agrochemicals in Kalpitiya area since there is no evidence of contamination of nitrate from alternative sources. Puttalam urban area too is affected with high nitrate contamination, which was undoubtfully proved from the microbiological testing as due to poor sanitation conditions. Concentration of nitrate in groundwater is increased by at least 1 to 5 mg/l during the dry season. Phosphate contamination of groundwater in deeper aquifers could also be identified at upper stretches of Vanathavillu (Eluwankulama, Rahalmadu and Serakkuliya) indicating it has more than permissible levels of 2 mg/l, although no such elevated levels were observed in shallow aquifers of the same area. This reveals that the phosphate contaminated groundwater could flow towards the deeper aquifers through fractures and weaker zones. Detailed investigations are required to confirm this hypothesis. However, an exceptionally high phosphate distribution was recorded in the Puttalam town area. Groundwater of the shallow aquifers of the surrounding area of Mee Oya shows higher electrical conductivity (EC) values than the permissible levels of 3500 µs/cm, although there is no indication of increased EC in groundwater of deeper aquifers (Figure 4). In some sampling points in the lagoonal areas as well as inland areas increased EC and salinity levels were observed and they are site specific characteristics. Distribution of EC in deep aquifers is observed in the 400 to 2000 µs/cm range except in the localized peaks where it rises to a maximum of 4000 µs/cm. such increased occasional EC values are identified in areas controlled by the characteristics of aquifer medium. This spatio-temporal behaviour of EC in the region can become more complex due to high heterogeneity of the subsurface layers within the same facies or formations. 132

5 Figure 2. Nitrate distribution of the shallow aquifers In Kalpitiya area, most of the land is covered by crop cultivations. The overburden of the area is found to be completely of sand underlain by limestone basement. The thicknesses of these sand beds are spatially varied and densely distributed. Various agricultural activities are observed on these sand beds. Farmers extract Figure 4. Distribution of electrical conductivity (EC) in the shallow aquifers Figure 3. Phosphate distribution of the deep aquifers high amounts of groundwater and do watering two to three times per day for their crops. Proper irrigation management systems such as drip irrigation, sprinkle system have not been practiced for these short term cultivations. It is important to record that these applications of fertilizers and pesticides are six times higher than the recommended levels, according to reliable sources of Irrigation Department. The elevated levels of nitrates and phosphates observed in this area are mainly due to infiltrating of agrochemicals into deep aquifers. A plot of Piper Tri-linear diagrams show that many shallow wells as well as deep wells fall into a zone of sea water composition (Figure 5). Groundwater quality of deep aquifers at Puttalam, Vanathavillu and Kalpitiya show predominantly Na-K-Cl type with some mixing towards Ca- Mg-HCO 3 type during both wet and dry periods in year. This is evidence to show that coastal regions are notably mixed with sea water. 133

6 Kumarasinghe, K.M.S.M. and Rajapakse, R.R.G.R. /Impact of anthropogenic activities on groundwater quality in Puttalam District Figure 5. Piper diagram showing the water quality type in the study area for both shallow and deep wells Several water-bearing limestone layers with variable thickness could be identified witin the unconsolidated sedimentary formation of the region as indicated by the available borehole data. Water chemistry and aquifer properties of limestone layers are heterogeneous. Thus, the groundwater quality of a source well largely depends on characteristics of each aquifer. Twodimensional survey was carried out to identify the internal structure of the aquifer and the coastal line of Vanathavillu area (~2 kms inland from the coast line) was illustrated, as an example (Figure 6). High resistivity values of the top most part of the Vanathavillu area indicates a dry sand layer by a clay rich layer as shown by low resistivity values. The bottom layer is higher in resistivity compared to the top clay layer indicating a limestone layer. It has a more complex structure with formations such as fault zone, cavities and clay-filled cavity zones etc. as confirmed by borehole logging data. Figure 6. 2-D resistivity image of Vanathavillu area 134

7 CONCLUSIONS Hydrochemical data of groundwater taken during both dry and wet seasons of Puttalam District in the Northwestern Sri Lanka show that both shallow and deep limestone aquifers have been contaminated with nitrate and phosphate due to various anthropogenic activities. Nitrate contamination pattern in groundwater occurs irregularly as lenses and pockets. However, there is a gradual tendency of increasing nitrate contamination throughout the Kalpitiya area. In this area, there is a tendency of nitrate contamination spreading out to surrounding aquifers. The distribution of nitrate variation in groundwater of the affected area should also be studied with respect the cultivation type, fertilizer type and the seasonal variation to determine the most valid reasons for nitrate contamination in the area. Phosphate contaminations in deeper groundwater aquifers could also be identified in the northern part of the study area. The geophysical applications of onedimensional and two-dimensional resistivity imaging surveys along with test well drilling, water level elevation contouring and aquifer test results may not be sufficient to trace the possible boundaries of salt-water intrusions due to complex regional geological and structural settings. Hence, it is recommended to identify spatial distribution of different aquifers and also implement groundwater management plan in these highly vulnerable sand and limestone aquifer formations in and around Puttalam District. ACKNOWLEDGEMENTS We gratefully acknowledge the assistance and help given by all supporting staff of Water Resources Board. REFERENCES Cooray, P. G. (1994). The Precambrian of Sri Lanka: a historical review: Precambrian Research, Vol. 66, p Cooray, P.G. (1984). Geology of Sri Lanka (Ceylon), National Museum of Sri Lanka publication, pp.184. China Water Supply Project in Puttalam (1987). The exploration and well drilling report on water resources in Chilaw Sri Lanka, The exploration team of Sri Lanka July 30, 1987 Jayasingha, P., Pitawala, A. and Dharmagunawardena, H.A. (2013). Fate of urea fertilizers in sandy aquifers: laboratory and field case study from Kalpitiya, Sri Lanka, J. Natn. Sci. Foundation Sri Lanka, Vol. 41(2), p Jayasingha, P., Pitawala, A. and Dharmagunawardhana, H.A. (2011). Vulnerability of coastal aquifers due to nutrient pollution from agriculture: Kalpitiya, Sri Lanka. Water, Air & Soil Pollution, Vol. 219(1-4), p Kuruppurachchi, D.S.P. and Fernando, W.A.R.N. (1999). Impact of agriculture on groundwater quality: leaching of fertilizers to groundwater in Kalpitiya Peninsula. Journal of Soil Science of Sri Lanka, Vol. 11, p Lawrence, L.R. and Kuruppuarachi, D.S.P. (1986). Impact of agriculture on ground water quality in Kalpitiya, Sri Lanka- implications for future development. British Geological Survey Report WD/OS/86/20. Liyanage, C.E., Thabrew, M.I. and Kuruppuarchchi, D.S.P. (2000). Nitrate pollution in ground water of Kalpitiya; an evaluation of the content of nitrate in the water and food items cultivated in the area. Journal of the National Science Foundation of Sri Lanka, Vol. 28(2), p Mubarak, A.M., Gunawardhana, H.P.G., Abeyratne, D.J., Kuruppuarachchi, D.S.P., Fernando, W.A.R.N., Lawrence, A.R., Stuart, M.E. and West, J.M. (1992). Impact of agriculture on groundwater quality: Kalpitiya Peninsula, Sri Lanka.Hydrogeology Series, Technical Report WD/92/04. Final Report: British Geological Survey. Wetselaar, R., Fox., J.J., Smith, G,D., Ali, R. M., Moermanto R.J. and Irdam Ahmad (1993). Groundwater nitrate in east Java. Indonesia. Journal of Australian Geology and Geophysics, Vol. 14, p World Health Organization (1984). Guidelines for drinking water quality Vol.1: Recommendations. Geneva. pp