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ANALYSIS OF NUTRIENT, SEDIMENT, AND PESTICIDE DATA FROM THE SOUTH PLATTE RIVER BASIN, WATER YEARS 1980-92

National Water-Quality Assessment (NAWQA) Program
South Platte River Basin Study

by Kevin F. Dennehy 1

ABSTRACT

The South Platte River basin was one of the first 20 study units selected in 1991 for investigation under the U.S. Geological Survey's National Water-Quality Assessment (NAWQA) program. One of the first major components of the South Platte study was a compilation, screening and interpretation of available nutrient, sediment, and pesticide data from surface- and groundwater stations in the basin. The retrospective analyses of existing water-quality data will provide an historical perspective on the water quality in the South Platte River basin, strengths and weaknesses of available information, and implications for water-quality issues, study priorities and study design.

The South Platte River basin drains about 24,300 square miles in parts of three States, Colorado (79 percent), Nebraska (15 percent), and Wyoming (6 percent). The majority of the basin population (approximately 2 million) is concentrated along the Front Range urban corridor in Colorado. The South Platte River originates in the Rocky mountains of central Colorado and flows about 450 miles northeast across the Great Plains where it joins the North Platte River at North Platte, Nebraska. Average annual precipitation varies greatly between the mountains and the plains. In the mountains, precipitation ranges from 7 to 60 inches, while in the plains average annual precipitation ranges from about 12 to 16 inches. The South Platte River flows from its headwaters along the Continental Divide through urban areas and onto the agricultural areas in the plains. The river is highly regulated along its entire length. Large quantities of water are diverted to ditches and reservoirs. Flows in the river, especially during fall and winter (low flow), are maintained by groundwater return flows from agricultural lands while during spring and summer (high flow) flows are dominated by snowmelt runoff and irrigation.

Three aquifer systems were examined in this study: South Platte alluvial system, High Plains aquifer, and the Denver Basin aquifer system (Dawson, Denver, Arapahoe, and Laramie-Fox Hills).

Most of the data analyzed were collected by the U.S. Geological Survey and the U.S. Environmental Protection Agency. Additional surface-water data were collected by the Denver Regional Council of Governments (DRCOG) and the Metropolitan Waste Water Reclamation District (MWWRD). Supplemental Groundwater data were included in the analysis from the North Front Range Water Quality Planning Association (NFRPA). After initial compilation of data from many sites, available data from 63 surface-water sampling sites and 110 wells were determined suitable for the intended purposes. The quantity of data available for these sites varied among the groups of constituents and media examined. For example, in surface water, more than 2,500 samples were analyzed for nutrients, 457 samples were analyzed for suspended sediment and 559 samples were analyzed for pesticides. With respect to groundwater, 410 samples were analyzed for nutrients and 20 samples were analyzed for pesticides. All the available data that were used were in machine-readable format.

The nutrients examined included: total nitrogen, dissolved organic nitrogen plus ammonia, dissolved nitrite, dissolved nitrite plus nitrate, total phosphorus, dissolved phosphorus, and dissolved orthophosphate. Sediment constituents examined were total suspended-sediment concentration and total suspended particle size. Pesticides examined include the following compound classes: organochlorine, organophosphorus, triazine and other nitrogen- containing compounds, carbamates, and chlorophenoxy acid compounds.

The median concentration of dissolved nitrite plus nitrate, as nitrogen, in surface water ranged from less than 0.01 to 4.8 mg/L and the median concentration of total phosphorus ranged from 0.01 to 2.55 mg/L. Point sources in urban areas, particularly waste water treatment plants (WWTP), showed an increase in nitrogen and phosphorus concentrations downstream of their discharges. There were no measurable increases in nitrogen and phosphorus species in agricultural areas in the lower part of the basin. Major tributaries showed similar increases in nitrogen and phosphorus through Front Range urban areas (WWTP discharges) to their confluence with the South Platte river. Most nutrient concentrations exhibited no temporal trends; however, flow-adjusted concentrations of organic nitrogen plus ammonia and dissolved phosphorus increased slightly over time downstream from Littleton, Colorado and Loveland, Colorado, respectively. Furthermore, concentrations of dissolved nitrite plus nitrate, dissolved ammonia, and dissolved phosphorus in agricultural and mixed agricultural/urban land use areas were significantly higher during low-flow season (fall-winter) than at other times in the year. These higher concentrations probably reflect the input of groundwater return flows that were not diluted with high surface-water flows during this period of the year.

Estimated annual nutrient loads were calculated for selected sites in the basin. Generally, organic nitrogen composed most of the nitrogen load at urban sites whereas nitrate composed most of the load at agricultural sites. Data were insufficient to calculate loads for many parts of the basin and also were difficult to interpret because of the complex water management practices. Surface water discharge, concentrations of dissolved nitrite plus nitrate and dissolved phosphorus decreased along the South Platte River from Kersey, Colorado to North Platte, Nebraska.

Nutrient concentrations in groundwater samples varied by aquifer, land use, and well type. No seasonal trend was observed. Concentrations of dissolved nitrite plus nitrate and dissolved ammonia were highest in the South Platte alluvial aquifer where the majority of the total groundwater samples were collected. The median concentration of dissolved organic nitrogen plus ammonia, dissolved nitrite, and dissolved nitrite plus nitrate in the alluvial aquifer were much higher than for any of the other aquifers. Organic nitrogen plus ammonia concentrations were highest in irrigation wells. Observation wells completed in the alluvial aquifer in rangeland areas had more than twice the median nitrite plus nitrate concentration than similar wells in agricultural land use areas. Only concentrations of nitrite plus nitrate indicated a trend with depth in the South Platte alluvial aquifer, concentrations decreased with increasing depth.

Overall, there is a lack of sediment data within the basin. Only four sediment sample sites were available along the South Platte River for analysis, but at least those sites were fairly well distributed areally. Tributaries to the South Platte River had little or no spatial coverage. The temporal distribution of sediment data was highly variable, with some sites sampled intensively for short periods, whereas other sites were sampled irregularly. Only the four main stem sites had an adequate distribution of samples by deciles of flow. Suspended sediment was not sampled frequently enough to enable analysis of loads, concentration trends, or particle-size trends. Three out of the four main stem sites are located in the agricultural land use areas, therefore more samples were collected in that land use area than in any other. Generally, suspended sediment concentrations increased from the foothills of the Rocky Mountains to the confluence with the North Platte River in Nebraska. However, at a number of sites along the South Platte River, the flow is diverted to reservoirs and ditches. This reduces the flow in the river, enabling large quantities of sediment to be deposited at these sites. It is clear that diversions affect or control much of the streamflow and sediment transport along the river. Suspended-sediment concentrations in the South Platte basin were much higher in agricultural areas than National Water Summary averages and much lower for rangeland areas. Suspended-sediment concentrations varied by month; most were transported by snowmelt runoff.

Pesticide data were available from 30 surface-water sites in the basin between 1980-92. However, approximately half of those sites are limited to 4 specific locations in the basin. Pesticide data for surface-water sites in the basin are limited by the number of analyses per station, the distribution of analyses by compound class and land use, the distribution of analyses throughout the year, the distribution of analyses by deciles of flow, and the number of analyses for the most heavily applied pesticides. Pesticide data in groundwater are available from 20 wells in the basin, none of which had more than one analysis. The areal distribution of wells is poor; most samples were collected from observation wells screened in the alluvium that underlies agricultural areas.

Most of the pesticide concentrations were less than laboratory reported detection limits. Generally, percent detections was highest for range and agricultural areas, somewhat lower for urban land use areas, and zero percent for forest land-use areas. Concentrations of 2,4-D decreased as discharge increased at those wells with sufficient concentration and flow data to examine this relation. Concentrations of CDPA, triazines, and other nitrogen-containing compounds were detected in groundwater, all detectable concentrations for atrazine and its metabolites were from wells in agricultural land-use areas.

1 U.S. Geological Survey, Box 25046, MS 415, Denver Federal Center, Lakewood, CO 80225

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