Scotts Bluff National Monument - Water Quality

In March 1999, the National Park Service Water Resources Division completed a baseline water-quality data inventory and analysis of Scotts Bluff National Monument. The primary goal was to provide a descriptive water-quality inventory which would be useable by the national monument’s managers. The results indicated that more water-quality information was needed. The United States Geological Survey (USGS) in cooperation with the National Park Service began a 1-year study in 2001 to assess the water quality of the North Platte River near the national monument during low- and high-flow events and at Scotts Spring within the national monument.

Potential sources of water contaminants near the national monument include industrial and municipal discharges, agricultural activities, residential sources, quarrying and mining activities, and recreation. Previous work by the USGS and other researchers and agencies in the North Platte River near the national monument identified water-quality concerns including: fecal bacteria, nutrients (nitrogen and phosphorus), sulfate, semi- volatile organic compounds, polychlorinated biphenyls, and uranium. Historical water quality data is available for locations near the national monument.

In the national monument area most of the ground-water movement is towards the North Platte River. An important hydrologic process is the surface-water/ground-water interaction where ground-water aquifers have a direct hydraulic connection with the North Platte River. Thus, ground-water gradients and water quality are affected by flows in the river. Local indicators of surface-water/ground-water interaction may be specific conductance values, and concentrations of calcium, sulfate, and uranium.

Previous studies indicated that the water quality of the North Platte River fluctuates over time as does nearby ground-water quality. Generally, nitrate concentrations in the ground water are higher than natural background levels in areas where agricultural fertilizers and manure have been applied. Nitrate levels from the North Platte River were less than 3.0 mg/L (milligrams per liter). Near the North Platte River, shallow ground water usually was characterized by large nitrate concentrations but may have been diluted with river water that had low nitrate concentrations. The herbicides alachlor, atrazine, cyanazine, and metolachlor are the four most commonly applied and detected pesticides. Herbicides are commonly present in small concentrations in surface water. High concentrations of uranium in the North Platte River area are directly related to local geology.

Sampling and collection of discharge measurements of the North Platte River were done in May 2001, during high-flow conditions and in June during low-flow conditions. One additional sample from the North Platte River was collected in September at the end of the irrigation season. Depth-width integrated water-quality samples from the North Platte River were analyzed for turbidity, major ions, selected trace elements, nutrients, herbicides, concentrations of uranium, chlorophyll, and fecal coliform bacteria.

Water samples from Scotts Spring were collected in May to be analyzed for turbidity, major ions, selected trace elements, nutrients, uranium, hydrogen and oxygen isotopes, and fecal coliform bacteria. Field measurements from both the North Platte River and Scotts Spring included specific conductance, pH, temperature, dissolved oxygen, and alkalinity. Water-quality samples were analyzed at the USGS National Water Quality Laboratory in Arvada, Colorado. Bacteria samples were analyzed at the Enviro Service, Inc. laboratory in Scottsbluff, Nebraska.

Measurements taken during the North Platte River surface-water sampling in 2001 for this study indicated specific conductance ranged from 818 to 900 µS/cm. The pH values in the surface-water samples ranged from 8.0 to 8.4. The dissolved-oxygen concentrations were found to be 8 to 9.1 mg/L. Major ions of sodium and sulfate dominate the surface-water samples from the North Platte River. In the surface water, nitrite/nitrate and ammonia concentrations were less than 3.0 mg/L. Herbicides were detected in all of the surface-water samples. Uranium concentrations in the surface water varied from 0.28 to 12.1 pCi/L (picocuries per liter), which is less than the Maximum Contaminant Level (MCL0 0f 30 pCi/L.

Scotts Spring had a specific conductance of 308 µS/cm, pH value of 8.4, and dissolved oxygen concentrations of 9.2 mg/L. Major-ion chemistry was dominated by silica and sodium in the spring, which overall resembles ground-water quality from the Chadron aquifer in the area. In the spring water, nitrite/nitrate and ammonia concentrations were near or less than the reporting limit. Samples were not analyzed for pesticides at Scotts Spring. The uranium concentrations ranged from 0.03 to 1.57 pCi/L which is less than the MCL of 30 pCi/L.

During evaporation and condensation of water, the concentration of oxygen and hydrogen isotopes that make up the water molecule undergo small changes which can be measured with great precision. The history and pathway of water in different parts of the hydrological cycle can be traced by following the abundance of the stable heavy isotopes of hydrogen (d 2 H) called deuterium, and oxygen (d 18 O). In this way, water in different environments develops isotopic “fingerprints” with which it can be identified and its origins traced. Isotopic techniques can help researchers assess the vulnerability of ground water to pollution from the surface by indicating how rapidly it moves and where it is being recharged. In oxygen and hydrogen isotopic results the differences in ratios between the sample and the standard are very small. They are expressed as parts per thousand or ‘per mil’ (‰) deviation from the standard (Coplen, 1994). These ratios are often used to provide information about water, and recharge sources of an aquifer. Isotopes of d 2 H and d 18 O sampled at Scotts Spring were -94 and -12 mil, respectively. The isotopic data indicate that the water may not have originated from the North Platte River, but from the Chadron aquifer.