INTRODUCTION
Nitrate is one of the most common contaminant found in groundwater. The principal sources of nitrate in groundwater are associated to agricultural activities (inorganic and organic fertilizers) and animal and domestic waste (Andersen and Kristiansen, 1984; Spalding and Exner, 1991). The presence of high nitrate concentration above the permissible level of 45 mg/L can cause significant problems in aquifers that are been use for water supply for urban areas. This is the case of the aquifer underlying the City of La Plata that is been pumped to supply potable water to this city.
Today, half the total potable water (100 hm3/year) is provided by groundwater from wells located in the urban area (50 hm3/year). However, most of the groundwater present nitrate concentration above the permissible level of 45 mg/l in Argentina. Based on the existence of about 20,000 hectares of agricultural land in the vicinity of the city, it was postulated that the nitrate was related to the high use of fertilizers in these area. However, a detailed evaluation of the groundwater flow system and the nitrate pattern in the study area, suggested that the source of the groundwater nitrate is related to sewage water leaking from the sewage network and old septic systems.
One of the common tracers used in groundwater studies to evaluate sources of nitrate and processes that affect nitrate concentration along the groundwater flow system is nitrogen-15 (15N), an stable isotope of the nitrogen (Kreitler and Browing, 1983; Wassenaar, 1995; Bohlke and Denver, 1995; Aravena and Robertson, 1998). This paper discuss 15N data on groundwater nitrate collected from rural and urban areas to provide more information about the sources of nitrate in the La Plata aquifer.
STUDY AREA
The study area is located in the extreme NE of the Buenos Aires Province. The city of La Plata is located in the center of the study area, that occupies an area of about 105,000 hectares. This area is part of the SE border of the great Chacopampeana Plain that has an extension of about one million of km2 in Argentina.
Go to Figure 1
The total population is about 665,000 habitants with about 640,000 concentrated in the cities of La Plata, Berisso and Ensenada. The rest of the population (25,000) is located in the rural areas related to agricultural and livestock activities.
The average precipitation (1909-92) in the study area was 1,020 mm. The rainiest month was March (110 mm) and the less rainy one was June with 62 mm. The average annual temperature was 16°C, with the hottest (22.5°C) and colder (9.7°C) months been June and July, respectively.
HYDROGEOLOGY
The main geological units present in the study area are the Postpampeano, the Pampeano and the Puelche formations (Figure 2).
The Postpampeano is the most recent formation (Upper Pleistocene and Holocene) and consists of clays, silt and sand of marine, fluvial and lacustrine origin, with thickness of few centimeters in the high part of the study area to about 25 meter close to the River La Plata (Figure 2). This unit basically functioned as an aquitard and is characterized by the presence of high salinity groundwater.
Go to Figure 2. Hydrogeologic Section
The Pampeano Formation (Medium-upper Pleistocene) underlain the Postpampeano in the coastal areas and the soil in the high plain areas. This unit is formed by silty sand of eolian and fluvial origin. Its thickness. varies between 0 and 50 meters and play a role in the recharge and discharge of the lower Puelche aquifer. This aquifer is the most important in the study region and is formed by medium and fine sand of fluvial origin.
The Puelche aquifer is used to supply water to the city of Buenos Aires (10 millions people), to the city of La Plata and for irrigation and industrial purposes. The thickness of this aquifer varies between 20 and 30 meters. Its average transmissivity, based on a permeability of 25 m/day and a thickness of 20 m, is 500 m2/day. Very little information is available for the formations underlain the Puelche, since they are characterized by the presence of high salinity groundwater.
The groundwater flow system is toward the Rio de la Plata and a clear depression cone is observed in the City of La Plata, due to heavy pumping (Figure 3).The groundwater velocity varies between 5 mm and 1 m/day and between 2 cm and 6 m/day for the Pampeano and Puelche aquifers, respectively (Auge et al., 1996). The groundwater flux to the depression cone is around 107,000 m3/day (39 hm3/ year ).
The groundwater extraction from the Puelche aquifer is about 50 hm3/year. Therefore, there is a difference of 11 hm3/year, that is compensated by leakages from the water supply and sewage systems, that has been estimated to be about 15 hm3/year (Auge et al., 1996). The groundwater geochemistry basically show the dominance of sodium bicarbonate water type in the aquifers changing to sodium chloride in the high salinity coastal areas (Auge, 1995).
Go to Figure 3. Potentiometric surface, contour interval is 5m
MATERIALS AND METHODS
A selected number of wells representative of the agriculture, live stock and urban areas (production wells) were sampled in this study. These wells include shallow and deep ones, representative of the Pampeano and Puelche aquifers, respectively. Major ions and N-15 analyses were performed on the groundwater samples.
The water chemistry was done at the UBA Laboratory, University of Buenos Aires and INA Laboratory using standards analytical techniques. The N-15 analyses were performed at the Environmental Isotope Laboratory, University of Waterloo. The N-15 analysis consists on the extraction of nitrate in anion exchange resins, nitrate elution, preparation of AgNO3 using Ag2O, and then combustion of the dry AgNO3 in a quartz breakseal at 850°C using a Cu/CuO mixture. The resulting N2 gas is later analyzed in a VG PRISM mass spectrometer with an analytical error of ±1.0 ‰. The N-15 analyses are reported in δ permil units defined as follow:
δ15N = (RS /RST -1) x 1000
where RS and RST are the 15N/14N ratio of the sample and standard, respectively. The international standard used for N-15 is AIR.
RESULTS AND DISCUSSION
On a regional scale and considering the groundwater flow direction toward the city of La Plata, the nitrate pattern shows some point sources of high nitrate concentration (~260 mg/l) in the upgradient areas in the middle of relative low nitrate (< 45 mg/l) areas. this trend changes to values between 67 and 13 mg/l in the intermediate areas to values as high as 100 mg/l in the wells 1ocated in the middle of the la plata city (figure 4). this pattern have been documented by auge and nagy (1996) as part of a major hydrogeological and geochemical study in the la plata aquifers (auge, 1995).
Go to Figure 4. Distribution of nitrate concentration and δ15N
The higher concentrations are observed in shallow wells located close to barnyards in areas with a high concentration of cattle stock. These wells are also close to septic systems. The higher nitrate concentration is clearly associated with the input of nitrate from animal and domestic waste. These are point sources of nitrate contamination. However, on a regional scale, these sources are not significant. Live stock activities are not extensive in the study area, and the normal practice is to have a few animals per hectares. A representative well (Puelche aquifer) of the live stock area and located in the outskirt of the Bavio City show a nitrate concentration of 13 mg/L. Therefore, the input of nitrate from animal waste at a regional scale from the live stock areas should be minimal.
The influence of point sources, in this case associated to the poultry industry (Well 493, 162 mg/L) and septic system (Well 224, 100 mg/L) is also observed in agriculture areas. Well 224 is a shallow well (50 m) located closed to two septic systems. Wells (402 and 173), representing the agriculture areas, show a nitrate range between 29 and 58 mg/L (Figure 4).
The production wells located upgradient (M39, M47, M41) and in the periphery (M54, M46, S38) of the City de la Plata show nitrate values between 13 and 41 mg/L. The two wells sampled in this study inside the city (M13 and S2) show nitrate values greater than 100 mg/L. The nitrate pattern observed in this study agree with data reported by Auge and Nagy (1996) and support the hypothesis that the main source of nitrate to the city of La Plata is leaking from the sewage network and old septic systems that are still in operation. The 15N component of this study was designed to further test this hypothesis.
The 15N data show a significant range of δ15N values that varies between + 8 and 20 ‰. The more enriched δ15N values are observed in the groundwater characterized by the highest nitrate concentration (~260 mg/L) in the cattle areas (Figure 4). These values are typical of nitrate produced from animal waste (Wassenaar, 1995) and is in agreement with barnyards as sources of nitrate.
The nitrate associated with agriculture area show δ15N values between 10.1 and 11.1 ‰. Groundwater in the agriculture areas affected by the input of septic system (well 224) and poultry industry (well 493) show δ15N values of 14.4 and 15.9 ‰, respectively.
Most of the production wells located upgradient (M39, 47 and 41) and in the periphery (M54 and S38) of the City de La Plata, show δ15N values around 8.5 and 10.9 ‰. The 15N data and the nitrate concentration lower than 45 mg/L suggests a nitrate contribution from the agriculture areas. The highest nitrate groundwater with concentration ranging between 70 and 100 mg/L (wells M28, M13 and S 2) show δ15N values between 13.5 and 14.5 ‰ (Figure 4). The increase on 15N content and nitrate concentration observed in the La Plata groundwater in comparison with the upgradient groundwater have to be associated with an input of nitrate from an in-situ source. Since nitrate derived from domestic waste are more enriched in 15N compared to agriculture sources, the in-situ nitrate source has to be leakage from the sewage pipeline and septic systems. Additional sampling for 15N analysis from the high nitrate areas will be performs to confirm the 15N pattern. In conclusion, the nitrate concentration and the 15N pattern indicate the high nitrate concentration observed in the City of La Plata groundwater above the permissible level is associated with leakage from the sewage network and septic systems. Agriculture areas are the main source of nitrate for the groundwater being pumped in the production wells located upgradient of the city of the La Plata.
REFERENCES
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