Ground Water Monitoring Activities Arkansas River Valley Alluvial Aquifer 94-95

Home Browse Search

If - 0279 EXECUTIVE SUMMARY The Water Quality Control Division (WQCD) of the Colorado Department of Public Health and Environment (CDPHE) has responsibility under the Agricultural Chemicals and Ground Water Protection Program (SB 90 -126) to conduct monitoring for the presence of commercial fertilizers and pesticides in ground water. This data assists the Commissioner of Agriculture in determining whether agricultural operations are impacting ground water quality. This report describes the monitoring program for groundwater quality in one of Colorado's major agricultural regions, the Arkansas River Valley. In 1994 -95 the program monitored groundwater quality in one of Colorado's major agricultural regions, the Arkansas River Valley. The program sampled one hundred thirty nine (139) domestic, stock, and irrigation wells throughout the valley (Figure 1). Each well was sampled once between July and December, 1994. Well samples were analyzed for basic constituents, dissolved metals, and selected pesticides (Table 1). The laboratory results and field data from the survey have been entered into the CDPBE Ground Water Quality Data System. Analysis of the laboratory reports, particularly for the nitrate and pesticide data, indicates that ground water in several areas of the study has been impacted by various agricultural chemicals. The major inorganic contaminant of concern is nitrate. Nineteen of 139 (14 %) of the wells sampled, showed nitrate levels in excess of the EPA standard for drinking water (10 mg/L) (Figure 2). The majority of the wells that exceeded the nitrate standard were located in Otero County (Figure 3). Twelve of 139 ( 9 %) samples showed positive for the herbicide Atrazine. One sample detected the herbicide Metolachlor and one sample detected the herbicide 2,4 -D (Figure 4). All pesticide detections where well below the drinking water standard. In August 1995, a confirmation sampling program was conducted to confirm pesticide detections and elevated nitrate levels measured in 1994. The confirmation sampling program consisted of resampling thirty two (32) wells that had shown either a pesticide detection or nitrate level above the standard in the 1994 sampling. The 1995 results confirmed the quality of the 1994 field and laboratory work. Of the nineteen (19) wells that had nitrate levels in excess of the EPA drinking water standard of 10 mg/L in 1994, four decreased below the standard. Two wells resampled for a pesticide detection increased above the standard. The pesticide resampling showed four wells with a trace amount of Atrazine (up to but not over 0.49 ug/L) in 1994, falling below the 1995 detection limit of 0.1 ug/L. One well resampled for nitrate picked up a hit of Atrazine. Another well increased from a trace level to 4.20 ug/L which is over the standard of 3.0. This is the only occurrence of pesticides at or above a water quality standard in the Arkansas Valley survey. This report provides the details of the monitoring effort in the Arkansas River Valley alluvial aquifer to the Commissioner of Agriculture. Sections describing the area sampled, the protocol for sampling and analysis, and the results of the analysis are provided.