WSP01544

• File Number: 8170
• Description: Arkansas Basin Water Quality Issues
• State: CO
• Basin: Arkansas
• Water Division: 2
• Date: 1/1/1996
• Author: Bradford Austin
• Title: Report to the Commissioner of Agriculture - Colorado Department of Agriculture - Ground Water Monitoring Activities - Arkansas River Valley Alluvial Aquifer - 1994-1995
• Water Supply Pro - Doc Type: Report/Study

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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 I). Each well was sampled once between July and
December, 1994. Well samples were analyzed for basic constituents, dissolved metals, and
selected pesticides (Table I). The laboratory results and field data from the survey have been
entered into the CDPHE 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. NlI1eteen of 139 (14%) of the wells sampled showed nitrate
levels in excess of the EPA standard for drinking water (10 mgIL) (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 MetolachIor and one sample detected the herbicide 2,4-0 (Figure 4). All pesticide
detections where well below the drinking water standard.

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In August 1995, a confirmation sampling program was conducted to confinn pesticide
detections and elevated nitrate levels measured in 1994. The confinnation 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 mgIL 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 ugIL) 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 ofJ.O. 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.