1. INTRODUCTION
Nitrate is the most common contaminant of groundwater. It mainly originates from agricultural fertilizer application and release of sewage. The World Health Organization has set the maximum acceptable
concentration of nitrate at 10 mg N L-1 due to health concerns related mainly to methemoglobinemia (WHO
1984). Previous studies on the municipal water supply aquifers of the Costa Rican Central Valley have
indicated nitrate levels are steadily rising and that some wells are above or near the WHO standard
(BGS/SENARA 1988,; Rodriguez-Estrada and Loaiciga 1995).
The Central Valley, in Costa Rica s central highlands, is a topographical depression at an elevation of about
1000 meters above sea level (masl) surrounded by volcanoes. The soils in the area are very fertile and, as
a result, Costa Rica has the highest production of coffee per hectare of any nation in the world (Reynolds-
Vargas and Richter 1995). The northern and the central parts of the Central Valley were formed with volcanic material from the Quaternary Period (Fernandez 1969). The aquifers, comprised primarily of fractured basaltic lava, are interspersed with tuffs (volcanic ash), which tend to behave as aquitards (BGS/SENARA 1985 and 1988). Groundwater supplies drinking water to about 50% of the Central Valley s rapidly growing population of almost 2 million. Groundwater in Central Valley is vulnerable to contamination from surficial sources due to the high annual precipitation (2000 to 5000 mm) and the relatively high permeability of the unsaturated zone (Reynolds-Vargas et al. 1995).
The two major sources of nitrate contamination in the Central Valley are the intense application of fertilizers used in coffee cultivation and in the disposal of untreated municipal wastewater into rivers and/or directly to the ground surface. In the Central Valley it is unclear whether the nitrate observed in the municipal wells originates from coffee cultivation or domestic sewage/urban runoff, or both. The predominant pathway by which the nitrate enters the groundwater system and travels to the municipal wells is also unclear. The geologically complex system is comprised of interlayered fractured aquifers separated by discontinuous aquitards. This creates a complex groundwater surface water interaction where rivers are known to be influent (gaining groundwater) or effluent (losing to the groundwater) along their reaches. The influent and
effluent stretches of the river are strongly affected by the geology. Effluent stretches tend to occur when
aquifer-aquitard contacts outcrop along the river valley. Given the distinct wet (May to November) and dry
(December to April) seasons, seasonal variations in the location and influent and effluent sections may also
occur. Nitrate-rich water may enter aquifers either through direct recharge to unconfined aquifers and/or
via direct runoff into streams and rivers followed by seepage into the groundwater at a lower elevation. It is
also likely that upper aquifers leak downward into lower aquifers in some locations, leading to even deeper
migration of nitrate.
The goal of this study is to understand the origin and pathway of nitrate in the municipal aquifers. Given the appreciable size of the Central Valley, a representative watershed was studied. The Mancarrón watershed is located in Heredia directly above many of the municipal wells and springs. It is typical hydrologically and in terms of land-use (natural forest in the upper elevations, coffee cultivation in the middle and lower elevations and urbanization in the lower elevations). The specific objectives of this study are to: (1) Observe the nature of groundwater-surface water interactions along the river path; (2) Determine the origin and pathways of the nitrate found in the groundwater.
2. METHODS
2.1 Site Description
Located in the northern part of the Central Valley, the Mancarrón River watershed as an area of 12.5 km2
(Figure 1b). The river has a length of 12.7 km, an average slope of 10.6% and a single tributary, the Quebrada Salas. The watershed elevation varies between 2340 and 1100 masl. Annual precipitation within the watershed ranges from about 2350 mm in its lower part to 3000 mm in the highest part (IMN 2000) and occurs mainly from May to November. Deep and well-structured Andisol soils in the region are derived from weathering of volcanic ash. They are dominated by Udands (previously classified as Dystrandepts; Peres et al. 1978) with spatially variable amounts of amorphous material and degree of weathering (Ryan et al. 2001). The Mancarrón River is incised into lava from an elevation 2000 m until 1300 m and in weathered tuffs in the remaining sections (Figure 1d).
2.2 Land use
The watershed is divided in four stretches related to the river sampling stations. The highest section, from 2340 to 1900 masl, is located between the river source and the highest river sampling station, R1 (Figure 1c). The second section, from 1900 to 1600 masl, is located between R1 and R2. The third section, from 1600 to 1350 masl, is located between R2 and R3. The lowest section, from 1350 to 1100 masl, is located between R3 and R4. Land use assessments have been conducted with a 1:10 000 land use map (Table 1; IGN 1991).
Go to Figure 1 a) Location in Costa Rica b) Watershed limits c) Land use and sampling stations d) Top soil distribution map
There are three major towns in the watershed (Figure 1b). They include San José de la Montaña, at an
elevation of 1520 masl (population 2782 in 1985); San Pablo (1260 masl, population 3048); San Pedro (1180 masl, population 4882). Coffee plantations are one of the major land uses in the watershed and these plantations have depended on increasing nitrogen fertilizer application over the past 30 years. The national average rate of nitrogen application in coffee plantation is around 270 kg/ha as N per annum (Reynolds-Vargas and Richter 1995). Fertilizers are generally spread in 2 to 3 applications of 90 kg/ha as N during the wet season usually in May or June, August, and October or November.
2.3 Hydrogeology
The Mancarrón River watershed was formed primarily during the Quaternary Period with materials derived
from the Poás, Barba and Irazu volcanoes (Fernandez 1969). The aquifers are located in the basaltic lava with high hydraulic conductivities primarily derived from secondary fracturing and weathering (BGS/SENARA
1985; 1988). The interlayered aquitards are primarily composed of volcanic tuff. The Barba Formation has a thickness up to 95 meters and includes several lava flows separated by aquitards (Figure 2). Two aquifers (Los Bambinos and Los Angeles) are exposed in its upper part. Several high elevation springs are located where aquifer-aquitard contacts outcrop. The major aquifer in the Barba Formation is Bermudez. This aquifer has a thickness up to 85 meters and is located underneath Los Bambinos and Los Angeles (BGS/SENARA 1985). Each aquifer is separated by the Porrosatí and Carbonal aquitards.
Go to Table 1
Go to Figure 2 Geologic cross section along the Mancarrón River valley.
Aquitard units are hatched. The Bermudez aquifer is exposed mainly along river canyons and undergoes major seasonal water table variations. It is known to have complex influent-effluent relationships with rivers (Darling et al. 1989). The Colima Formation is located below the Bermudez aquifer and includes two major aquifers, Colima Superior and Colima Inferior separated by an aquitard. Colima Superior and Bermudez aquifers are separated by the Tiribi aquitard which is known to be thin and have a high permeability in the area (Darling et al. 1989). Most aquifers in the Central Valley have high hydraulic conductivity with an average velocity is about 2m/day (Reynolds-Vargas and Richter 1995). The general direction of the flow of the Bermudez aquifer is NE to SW (BGS/SENARA 1985; 1988), along the topographic gradient.
2.4 Water Sampling and Analytical Methods
Water samples were taken from 3 wells, 1 spring, 3 rain gauges and 4 river stations (Table 2; Figures 1b &
2) located through out the area over a range of 800 masl in elevation. All the stations were sampled 40 times over a period of 43 weeks from June 22, 2000 until April 18, 2001, and five parameters were analyzed; nitrate, chloride, silica as well as water isotopes, δ2H and δ18O.
Nitrate samples were filtered through a 0.45 μm filter in the field and acidified with HCl to a pH~2 in the laboratory, within 6 hours of sampling and then frozen until transported to the University of Calgary. Silica samples were refrigerated until their analysis at the University of Costa Rica. The chloride and isotopes samples were shipped back to the University of Calgary for analyses with no treatment. Nitrate was analysed with cadmium-dye azo dye colorimetry using a Technicon Autoanalyzer. Chloride was analysed using the automated ferricyanide method (APHA, 1988) using a Technicon Autoanalyzer. Silica was measured using the molybdosilicate method follow by the heteropoly blue method (APHA 1988). Deuterium analysis was conducted by reduction of water hydrogen with zinc at 500ºC (Coleman 1982) using a VG 602 double collector mass spectrometer. Oxygen-18 analyses were carried out by the standard Epstein-Mayeda carbon dioxide equilibration method using a VG 903 triple collector mass spectrometer. Results are reported in standard δ notation relative to V-SMOW.
Rainfall samples for δ2H and δ18O analysis were collected in 3 rain gauges (see Figures 1b & 2 for location). A layer of liquid paraffin was used to prevent evaporation of the collected rainfall. The rainfall was sampled weekly but 4 to 5 weeks were mixed together to get a monthly average. The number of weeks mixed depended on the sampling date. River discharge was estimated using a graduated container and a chronometer. Shallow mini-piezometers were installed in September 2000 at each of the R3 and R4 sampling locations to observe if the river was influent or effluent.
3. RESULTS AND DISCUSSION
3.1 Precipitation Isotopes
An altitude effect in precipitation isotope ratios, with increasingly depleted values at increasing elevation, is
usually observed (Clark and Fritz 1997). The volume weighted mean isotopic composition of rainfall for the
10-month period at the three Mancarrón rain gauges shows a reverse altitude effect (Figure 3). This is
believed to result from the derivation of precipitation in the Central Valley from both the Pacific and Atlantic
oceans depending on the wind direction and season. Although precipitation from the Pacific coast likely
exhibits a regular altitude effect, it is apparently overridden by more enriched precipitation originating
from the Atlantic coast. Similar results have been found in other studies in the Central Valley (Hirata
2001). Given the reverse altitude effect, groundwater recharged at increasingly lower elevations would be
increasingly depleted in δ18O and δ2H This is contrary to earlier interpretations (BGS/SENARA 1985). An inverse relation between water isotope ratios and altitude is observed in the river and groundwater (see later sections) and was also observed in springs in the Central Valley (Darling et al. 1989).
Go to Figure 3 Average and standard deviation of δ18O and δ2H in precipitation sampled from different elevations
The local meteoric water line (LMWL) for precipitation sampled in this study δ2H = 8.5 δ18O + 14.7; Figure 4) agrees well with other studies (BGS/SENARA 1988; Darling et al. 1989). The LMWL of the three rain gauges (PI, P2 and P3) do not differ significantly.
Go to Table 2
The rain isotopic composition varies between the wet and dry seasons; with lighter ratios during wet season
and heavier during the dry season (Table 2). The month of May and three weeks in June are missing for
the year average. The monthly average rainfall in May and June in the area is significant (250 to 325 mm in
May and 260 to 330 mm in June; IMN 2000). Based on the lighter isotopic average of the rainfall during wet
season, it is clear that the year average would be slightly more depleted if a full year of analyses was
included in the calculation. The groundwater δ18O and δ2H (Figure 4) are slightly more depleted on average than the precipitation. This further indicates that the average isotopic composition of rain averages would be more depleted in May and June precipitation were included.
3.2 Isotopic Composition of Groundwater
Groundwater samples tend to fall into two fields on an δ18O versus δ2H plot (Table 2, Figure 4). More enriched values are found in the sampling points at higher elevation (W1 and S1). This water is derived from the Los Angeles and Los Bambinos aquifers, with infiltration apparently occurring at a similar elevation to the sampling points (Figure 3). Groundwater from the lower elevation sampling points (W2 and W3) completed in the lower Bermudez aquifer, tends to have more depleted δ18O and δ2H (Table 2; Figure 4). Given the reverse altitude effect, water recharging these wells is apparently precipitated at elevations that are lower than 1600 masl (Figure 3). The average δ18O and δ2H values for groundwater wells lie close to the LMWL, with some points both below and above the line (Figure 4). The deviation from the LMWL suggests the water is subject to processes that affect δ18O and δ2H values (e.g. evaporation and silicate hydration; Clark and Fritz 1995; Figure 4). Water from S1 lies slightly more above the LMWL than any of the wells (Figure 4), suggesting it is subjected to more silicate hydration.
Go to Figure 4 Isotopic composition of precipitation and groundwater
3.3 River discharge and isotopic composition
River discharge varies temporally and spatially along the river path (Figure 5). At the highest station (R1) located at an altitude of 1900 masl, a dam diverts river water into a distribution system for a local municipal water supply. Overflow from this diversion remains in the Mancarrón River. Additional river water is derived from a perennial spring located at 1770 masl. Overflow from the diversion was present only from August, 2000 to February, 2001. Throughout this period, river discharge increased with decreasing elevation until R3, with most of the increase occurring between R1 and R2 (Figure 5). Discharge then decreased between R3 and R4 (i.e. the river was effluent). After the dry season
was initiated in February, 2001, the river was effluent between R2 and R3, with a lower discharge measured
at R3 than R2. The river remained effluent between R3 and R4 throughout the year. Thus, in the stretch
between R2 and R3, the river is influent due to high groundwater tables in the wet season, but effluent
when water tables are lower in the dry season. The mini-piezometer located at R3 also indicated influent
conditions from August to February, with a reversal to effluent conditions after February.
Go to Figure 5 River discharge and monthly precipitation.
Go to Figure 6 Isotopic composition of river water
The river water isotope ratios become increasingly depleted between R1 and R2, with little subsequent change between R2 and R4 (Figure 6). It can also be seen that the river undergoes some evaporation as shown by the reduction in the slope value with respect to the LMWL (Figure 5, Table 3). The river water isotopes are similar to the groundwater, particularly at lower elevation (i.e. W2 and W3 (Figure 6). This suggests a significant fraction of the river water is derived from groundwater. Also, river water remains more or less constant and similar to groundwater through out the year while precipitation is seasonally variable (Figure 7). This also suggests a significant groundwater contribution to river water.
3.4 Rock-Water Interaction
Silica concentrations are low in precipitation (<1 mg/l; freeze and cherry 1979). they are increased in
groundwater with increasing contact with silica-rich soil and geologic materials. thus in a general sense,
higher silica values indicate increased subsurface residence times (hinton et al. 1994). this is consistent
with elevated silica concentrations in deeper groundwater in w2 and w3 (average values ~ 53 - 54
mg/l; table 2) and lower concentrations (37 - 41 mg/l) in the shallower groundwater in s1 and w1. in influent river stretches, an increase in silica concentration would occur with an increasing component of silica-rich groundwater. no change in silica concentration would occur in effluent river sections.
Go to Figure 7 River Isotopes variation with time.
Go to Table 3 Coefficients (m, b) from a linear regression of δ18O and δ2H in river water
Silica concentrations in the river are lowest at R1 (~8.8 mg/L), and increase to values that are not significantly different from one another in R2, R3 and R4 (23 - 24.6 mg/L; Table 2). The low silica concentrations at R1 as well as the enriched isotopes suggest that local runoff and/or very young groundwater supply the Mancarrón River upper reaches. The increase in silica concentration between R1 and R2 is consistent with influent conditions along this river stretch. A comparison of silica concentration in R2 with that of the shallow groundwater in this region (~37.8 to 41 mg/L; S1 and W1; Table 2) suggests that more than half of the river water is derived from groundwater. In the absence of information about groundwater silica concentration between R2 and R3 (where the river is alternately influent and effluent), the lack of significant change in silica concentration cannot be interpreted easily. Since the river is consistently losing between R3 and R4, the absence of significant change in silica concentration is expected.
The river water often lies above the LMWL on an δ18O vs. δ2H plot (Figure 6). This occurs most strongly in
R1. The displacement of data above the LMWL is consistent with silica hydration (Clarke and Fritz 1997).
The apparently higher degree of silica hydration at altitude (i.e. at R1) may be due to the increased
presence of younger, less hydrated tuffs at altitude, with a consequently higher capacity for silica hydration.
Go to Figure 8 Silica Variation Vs δ18O
Overall oxygen-18 isotope ratios and silica in the river water suggest it can be characterized by mixing between river water at altitude (R1) and groundwater (Figure 8). The occurrence of a number of data points below the mixing line may be due to limited subsurface transport (low SiO2 concentration) or increased δ18O depletion due to silicate hydration.
3.5 Nitrate and Chloride
The relation between nitrate and chloride can be a useful indicator of the source of nitrate in groundwater. A correlation between chloride and nitrate can indicate a sewage source (DeSimone and Howes, 1998; Zilkey, 2001), while elevated nitrate concentrations derived from commercial fertilizers would not have significantly elevated chloride concentrations. The natural chloride level in precipitation is between 1.1 and 1.8 mg Cl-/l (Table 2). Chloride concentrations in the river gradually increase with decreasing elevation (Table 2 & Figure 9, from an average of 0.6 mg/L at R1 to 3.3 at R4). These elevated chloride levels could be from domestic sewage, with nitrate assimilation in the river resulting in low nitrate concentrations.
Nitrate transport is usually considered to be conservative in aerobic groundwater. A correlation between increasing nitrate and chloride in S1 W1 suggest the water is being impacted by domestic sewage. The two lower elevation wells (W2 and W3), however, have clearly elevated nitrate without elevated chloride suggesting a fertilizer source of nitrate. As discussed previously, isotope values of W2 and W3 indicate groundwater is recharged at moderate elevations. The prevalence of coffee cultivation in these regions is consistent with a fertilizer source for elevated nitrate in W3 and W4. Given the highest nitrate concentrations are observed in W2 and W3, fertilizer nitrate impacts on groundwater appear to be more severe than domestic wastewater in the Mancarrón watershed.
Go to Figure 9 Chloride Vs Nitrate
The occurrence of only low levels of nitrate in the river relative to groundwater suggest that river infiltration into groundwater is not a significant source of groundwater nitrate. Rather, the nitrate appears to be derived from direct infiltration to unconfined aquifers.
4. CONCLUSIONS
The Mancarrón River is fed by local runoff and/or very young groundwater in its upper reaches. It is influent at moderate elevation (between R1 and R2). The river is alternately influent and effluent at mid-elevation
(between R2 and R3) depending on season, and is perennially effluent at lower elevation (between R3 and
R4).
The rain isotopic compositions show a reverse altitude effect. Higher watershed elevations have an enriched isotopic value compared to the lower elevations. This pattern is also observed in river and ground water and suggests a recharge area for the lower elevation wells occurs lower than 1600 masl, which is coincident with coffee cultivation.
The SiO2 and δ18O two-end member mixing model suggests groundwater constitutes more than half the river water at lower elevation. Both fertilizer and sewage impacts are indicated in the river. Sewage seems to predominate in S1, while lower elevation wells are apparently mainly impacted by fertilizers.
5. AKNOWLEDGEMENTS
We gratefully acknowledge funding from CIDA award for Canadians and the Central American Water Resource Management Network (www.caragua.org) and helpful discussions with Dr. B. Mayer and C.
Agudelo.
6. REFERENCES
- Agudelo, C.L., 2001. Evaluación de las concentraciones altas de nitratos en el manantial La Libertad y su relación con el campo de pozos La Valencia, Santo Domingo de Heredia, Costa Rica. Masters Degree
Thesis, Universidad de Costa Rica, San José, Costa Rica.
- American Public Health Association. 1998. Standard Methods for the Examination of Water and Wastewater. 20th ed; Section 9. (Greenburg AE, Clesceri LS, Eaton AD, eds). Washington, DC.
- BGS/SENARA (British Geological Survey and Servicio Nacional de Aguas Subterráneas, Riego y Avenamiento), 1985. Mapa hidrogeológico del Valle Central de Costa Rica. Scale 1:50 000. BGS/SENARA, San José, Costa Rica.
- BGS/SENARA, 1988. The continuation of hydrogeological investigations in the north and east of
the Valle Central, Costa Rica: Final Report 1984-1987. BGS Technical Report WD/88/13R. BGS/SENARA,
San José, Costa Rica.
- Clark, I. and P. Fritz, 1997. Environmental Isotopes in Hydrogeology. Lewis Publication, Boca Raton, New York. Coleman, M.L., T.J. Shepherd, J.J.Durham, J.E.Rouse and G.R. Moore, 1982. Reduction of water with zinc for hydrogen isotope analysis. Analytical Chemistry, 54, p. 993 - 995.
- Darling, W.G., J.M. Parker, H.V. Rodriguez and A.J. Lardner, 1987. Investigation of a volcanic aquifer
system in Costa Rica using environmental isotopes. Proceedings of regional seminar for Latin America on
the use of isotope techniques in hydrology, Mexico City, Mexico, p. 215 - 228.
- DeSimone, L.A. and B.L. Howes, 1998. Nitrogen transport in shallow aquifer receiving wastewater
discharge: A mass balance approach. Water Resources Research, 34 (2): 271 - 285.
- Fernandez, M., 1969. Las unidades hidrogeológicas y los manatiales de la vertiente norte de la cuenca del Río Virilla. Informe Técnico No. 27, Servicio Nacional de Aguas Subterraneas (SENAS), Costa Rica.
- Freeze, R.A. and J.A. Cherry, 1979. Groundwater. Prentice-Hall, Englewood Cliffs, N.J..
- Hinton, M.J., S.L. Shiff and M.C. English, 1994. Examining the contribution of glacial till water to storm
runoff using two and three-component hydrograph separation. Water Resources Research, 30: 983 - 993.
- Hirata, R., 2001. Personal Communication.
- IGN (Instituto Geográfico Nacional), 1991. Uso de la Tierra. Scale 1:10 000. IGN, San José, Costa Rica.
IMN (Instituto Meteorológico Nacional), 1999.
- Meteorological Station Files. IMN, San José, Costa Rica.
- Peres, S.R., E. Ramirez, and E. Alvarado, 1978. Mapa preliminary de asociaciones de subgrupos de suelos de Costa Rica. OPSA, San José, Costa Rica.
- Reynolds-Vargas, J.S., and D.D.Jr. Richter, 1995. Nitrate in groundwaters of the Central Valley, Costa
Rica. Environment International, 21 (1): 71 - 79.
- Reynolds-Vargas, J.S., L. Araguas, J. Fraile, L. Castro and K. Rozanski, 1995. Nitrate Leaching through
volcanic soils in the Central Valley, Costa Rica. Proceedings of an International Symposium on Nuclear
and related techiques in Soil-Plant Studies on Sustainable Agriculture and Environmental Preservation, Vienna, Austria, p. 549 - 559.
- Rodriguez-Estrada, H. and H.A. Loaiciga, 1995. Planning urban growth in ground water recharge areas:
Central Valley, Costa Rica. Ground Water Monitoring and Remediation. 15(3): 144 - 148.
- Ryan, M.C., G.R. Graham, and D.L. Rudolph, 2001. Nitrate adsorption in coffee plantation andisols. J. Env. Qual. In press. WHO (World Health Organization), 1984. Guidelines for drinking water quality. Vol. 1. WHO, Geneva, Switzerland.
- Zilkey, M.M., 2001. Fate of nutrients in groundwater under an irrigated, manured field, southern Alberta.
M.Sc. Thesis, University of Calgary.
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