Water Quality in the Las Vegas Valley Area and Carson and Truckee River Basins, Nevada and California, 1992-96

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MAJOR ISSUES AND FINDINGS

Agricultural Activities Have Been Sources of Irrigation Drainage, Nutrients, and Pesticides

Agriculture is a predominant land use in parts of the Carson River Basin (Carson Valley and Carson Desert) and of minor importance in the Las Vegas Valley and Truckee River Basin. Surface- and ground-water resources can be affected by irrigation drainage, nutrients (from fertilizers), and pesticides.  Irrigation Drainage Contributed Dissolved Solids, Arsenic, Boron, Molybdenum, and Uranium to Wetlands in Carson Desert Dissolved solids, arsenic, boron, molybdenum, and uranium are dissolved from desert soils by irrigation within Carson Desert. Previous studies [8,47] and one completed by the NVBR NAWQA in cooperation with the Department of the Interior National Water Quality Irrigation Program [9] have documented contamination of surface water by irrigation drainage in Carson Desert.

Irrigating crops in Carson Valley with treated sewage effluent, May 1996. Photograph by Michael S. Lico, U.S. Geological Survey.

Irrigated agriculture in Carson Desert, July 1989. Photograph by Michael S. Lico, U.S. Geological Survey.

Dissolved-solids, arsenic, boron, and molybdenum concentrations commonly exceeded aquatic-life criteria for water in Carson Desert wetlands [47]. These wetlands are an important area for migratory birds in the Pacific flyway. Concentrations of most constituents were greater in drainwater during the nonirrigation season (November-March) than during the irrigation season (April-October) because the flow is dominated by shallow ground-water discharge. Selenium does not appear to be an issue in irrigation drainage in Carson Desert, although it is in other parts of the Nation [48]. Loads (or quantities) of dissolved solids, boron, and arsenic delivered to wetlands in Carson Desert are important because evapotranspiration can greatly concentrate them and high concentrations are detrimental to aquatic life. Loads of chemical constituents generally were greater during the irrigation season than during the nonirrigation season [9]. The estimated yields (or load per unit area) of these constituents differed greatly among drain systems in Carson Desert (fig. 10). Yields are important because they help identify areas that are contributing most of the loads of constituents. Five specific areas have been identified as the greatest contributors of potentially toxic constituents to the drain systems [9].

Evidence Suggests Nutrient Enrichment in Some Agricultural Areas

Agricultural practices commonly include the application of fertilizer to increase plant growth and yield. Fertilizer can contain nitrogen and phosphorus in varying amounts. Less than 1 ton of nitrogen per square mile is applied to crops in western Nevada [49], in contrast to 7 tons or more per square mile in the upper Midwestern United States. Alfalfa, a legume that fixes its own nitrogen from the atmosphere, is the major crop grown in Carson Desert and does not need additional fertilizer after the plants are established. Animal manure also can contribute nitrogen to water in parts of the NVBR study area. An estimated 1 to 2 tons of nitrogen per square mile is contributed annually to Carson Valley by animal manure [49]. Less than 1 ton of nitrogen per square mile is contributed annually by this source in Carson Desert. These inputs of nitrogen are low compared to the rest of the Nation. Nutrients were enriched in the Carson River downstream from the Carson Valley and Carson Desert. Median concentrations of nitrate, total phosphorus, and orthophosphate from October 1969 through April 1990 were enriched at Carson River sites downstream from Carson Valley [28]. During most of that period, treated sewage was discharged into the Carson River (this practice was stopped in 1987); thus, agriculture in Carson Valley was not the only source of nutrients during that period. Concentrations of ammonia, total phosphorus, and orthophosphate in the Carson River upstream from Lahontan Reservoir (fig. 3) had decreased since the 1980s.

Figure 10. Loads of dissolved solids, boron, and arsenic delivered by agricultural drain systems in the Carson Desert during 1995 differed greatly.

A retrospective analysis of nutrient data collected from October 1969 through April 1990 [50] showed that shallow ground-water samples from beneath agricultural areas in the Carson River Basin had orthophosphate concentrations (median, 0.22 mg/L) that were greater than those from beneath urban areas (median, 0.04 mg/L for Las Vegas and Reno-Sparks combined). In comparison to other land uses (urban, range, and wetlands), ammonia and nitrate were not present in high concentrations beneath agricultural areas. In 8 of 45 shallow monitoring wells in agricultural areas, nitrate exceeded the MCL of 10 mg/L as N [7]. Ground-water samples collected from monitoring wells in the shallow aquifers during this cycle of NAWQA had median concentrations of nitrate of 0.16 and 0.66 mg/L as N for Carson Valley and Carson Desert, respectively. These concentrations were less than those in urban areas of the Study Unit [12].

 
Summary of pesticide compounds detected in surface- and ground-water and bottom-sediment samples in and near agricultural areas in the Carson River Basin, 1992-95

[Concentrations listed are maximum for that site and are expressed in micrograms per liter for water samples and micrograms per kilogram for bottom-sediment samples; --, no sample collected]

Pesticides Were Present at Low Levels in Agricultural Areas

Pesticides commonly are used by farmers to enhance the yield of crops by controlling insects, weeds, and fungi. The Carson River system has been somewhat affected by pesticide applications. A retrospective analysis of available data from October 1969 to April 1990 [51] showed that pesticides were present in the Carson River downstream from the Carson Valley agricultural area. Results from surface-water samples taken upstream from Lahontan Reservoir showed detections of 2,4-D, -BHC, chlordane, DDD, DDE, DDT, diazinon, endrin, heptachlor, and carbaryl. Samples collected from the Carson River upstream from Carson Valley agricultural area had only one pesticide detected. However, downstream from Carson Valley, but upstream from Lahontan Reservoir, six pesticides were detected in 12 water samples. Downstream from Carson Desert agricultural activities, eight pesticides were detected at two sites. Apparently, drainage from agricultural fields and other areas has contributed pesticides to the Carson River system, although at concentrations less than available MCLs.

Wetlands in Carson Desert, December 1988. Photograph by Michael S. Lico, U.S. Geological Survey.

Results of pesticide analysis for samples collected during 1995 from irrigation drains in the Carson Desert agricultural area [9] show the presence of pesticides in 17 of 19 samples. In all, 12 pesticides were detected. Drains at the downstream ends of their respective drainage systems tended to have water with a greater number of pesticides than those more upstream in the systems. Two of these downstream sites had six pesticides detected. Atrazine, simazine, and prometon were the most commonly detected pesticides. All pesticide detections were less than 1 µg/L except one value of 1.6 µg/L for 2,4-D. Concentrations detected did not exceed MCLs or aquatic-life criteria.

During this cycle of NAWQA, bottom-sediment samples were collected from surface-water sites to determine whether organochlorine pesticides had been contributed from upstream activities. Except for one sample, pesticides were not detected in bottom sediment collected in the Carson River. In this one sample, collected downstream from the Carson Desert agricultural area, DDE was detected. Previous investigators [22,47] reported detections of pesticides in bottom sediment collected from wetlands downstream from the Carson Desert agricultural area.

Shallow ground-water quality also may be affected by agricultural use of pesticides. During 1994-95, shallow ground-water samples were collected from 30 monitoring wells beneath agricultural areas in the Carson River Basin as part of the NAWQA study. Five samples from Carson Valley and three from Carson Desert had detections of pesticides at low concentrations (less than 1 µg/L), except for one sample that had 1.2 µg/L of atrazine, which was less than the MCL of 3 µg/L [7]. The most commonly detected pesticides were triazine herbicides, including atrazine, deethyl atrazine (a degradation product of atrazine), and simazine.

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