Nutrients include nitrogen and phosphorus compounds that are necessary components for the growth of plants and animals. However, in excessive concentrations, nutrients are a water-quality concern in drinking water and are a major contributor to eutrophication in rivers, lakes, and reservoirs. Large nutrient concentrations can contribute to excessive growth of algae and other aquatic plants that can cause destruction of habitat and depletion of dissolved oxygen, which usually results in the disappearance of intolerant aquatic insect species and fish. Major sources of nutrients are fertilizers, sewage effluent, precipitation, and dissolution of naturally occurring minerals (Mueller and Helsel, 1996, p. 10). Fertilizers, which include commercial fertilizers and animal manure, are applied in urban and agricultural areas. Sewage effluent includes municipal WWTP discharge to streams and wastewater from septic tanks or cesspools. An estimate of nutrient loads to the entire Study Unit indicated that loading from fertilizers and WWTP effluent is considerably larger than loading from precipitation (Anderholm and others, 1995, p. 127). As a result of human activities in the Rio Grande Valley Study Unit, nutrient concentrations in both ground water and surface water have exceeded established Federal and State standards.
The EPA has established an MCL for nitrate in drinking water of 10 milligrams per liter (mg/L) as nitrogen. The EPA recommends that total (not dissolved) phosphorus concentrations should be less than 0.1 mg/L in rivers and less than 0.05 mg/L where rivers enter lakes and reservoirs because concentrations greater than this could contribute to eutrophication. The States of Colorado, New Mexico, and Texas have also established individual aquatic-life criteria for selected nutrients that apply to specific river reaches within the Study Unit.
In the San Luis Valley agricultural land-use study, water from 11 of the 35 wells sampled contained nitrate concentrations greater than the EPA MCL; the largest concentration was 58 mg/L (table 6). In the Rincon Valley agricultural land-use study, water from 5 of the 30 wells sampled exceeded the EPA MCL; the largest concentration was 33 mg/L. These elevated nitrate concentrations are indicative of leaching of fertilizers into shallow ground water. However, the spatial variation in nitrate concentrations indicates that leaching of fertilizer is not uniform throughout these areas. The spatial variation may be the result of variable fertilizer application rates, timing of fertilizer application, timing of application of irrigation water, and recharge rates controlled by soil type, texture, permeability, precipitation, and biological/geochemical processes in the unsaturated zone.
In the Albuquerque urban land-use study, the largest nitrate concentration was 2.8 mg/L, which is considerably below the EPA MCL. Although infiltration of septic tank effluent results in considerable loading of nutrients to the shallow aquifer in this area, the small nitrate concentrations suggest that nitrogen compounds in the effluent are not being converted to nitrate. This is probably due to the lack of dissolved oxygen and the relatively large dissolved organic carbon concentrations in shallow ground water in the area. Large ammonia concentrations, which would be expected, are not present in shallow ground water because ammonia is adsorbed on clays in the aquifer or soil zone.
Nitrate concentrations in water from deep wells sampled during the aquifer subunit survey ranged from less than 0.05 to 1.9 mg/L. Throughout most of the area of the aquifer subunit survey, the small nitrate concentrations in water from deeper parts of the basin-fill aquifers probably indicate that shallow ground-water recharge into the deeper parts of the aquifer is limited.
Dissolved nutrient concentrations are shown in Table 7. The largest concentrations of dissolved nutrients in surface water were detected at sites downstream from urban land use. Generally, a WWTP was associated with the urban area and was located upstream from the sampling site. In general, dissolved nutrient concentrations were larger at Basic Fixed Sites downstream from Cochiti Lake. This is probably the result of the increase in urban land use along the Rio Grande and the associated WWTPs.
Dissolved nutrient concentrations in agricultural drains in the Mesilla and Rincon Valleys were small. These concentrations could be small because of biological uptake of nutrients and no widespread excessive nitrate concentrations in ground water that is discharging to drains. Comparison of data for the Mesilla Valley synoptic study shows that the inflow of nutrient loads to the Rio Grande from urban sources (WWTPs) may equal or exceed the inflow from agricultural sources (drains).
Of 455 samples at sites where aquatic-life criteria apply, un-ionized ammonia concentrations exceeded the applicable criterion only twice. No other dissolved nutrient concentration in surface water exceeded any applicable Federal or State criterion or standard.
The EPA has recommended total phosphorus concentrations for streams not discharging to a lake and for streams near where they discharge to a lake. These recommendations are not criteria or standards. Out of 526 samples from these sites, 250 equaled or exceeded the recommended concentrations.
Total nutrient concentration in surface water is affected significantly by suspended-sediment concentration because nutrients adsorb to suspended sediment and are transported by water. The largest concentrations of total nutrients were detected at the Basic Fixed Site on the Rio Puerco (fig. 2) and were associated with runoff from summer thunderstorms, which historically have transported large loads of suspended sediment.
Approximately 35 percent of the samples from the Basic Fixed Sites have total phosphorus and total nitrogen concentrations in the eutrophic or highly eutrophic range of most trophic classification systems. Among the environmental effects of eutrophication are large diurnal dissolved-oxygen (DO) fluctuations. In addition, the decay of large amounts of dead vegetation puts a demand upon available DO concentrations. During this study, DO concentrations less than the EPA recommended minimum ambient criterion of 5 mg/L for the health of an aquatic ecosystem were detected less than 7 percent of the time at sites in Colorado.
The effect that reservoirs can have on water quality is evident when dissolved and total nutrient concentrations are compared at sites upstream and downstream from reservoirs. Although not directly addressed during this study, historical data for sites on the Rio Grande show nutrient concentrations in water decreasing significantly while stored in reservoirs (Anderholm and others, 1995). The decrease downstream from reservoirs is attributed to biological uptake of nutrients and suspended sediment settling out of the water column.
Wastewater-treatment plants affect nutrient concentrations in the Rio Grande (photograph by Sherman R. Ellis, U.S. Geological Survey).
