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2 I I I I I' I I I ' I I I I' I' I I I I I I 028';) Based on the land use and hydrogeologic factors, the potential exists for migration of agricultural chemicals into the ground water in this area. During the preliminary planning for sampling, CDPHE contacted interested parties to inform them of the sampling program and SB 90-126, and how we envisioned its implementation. CDPHE has coordinated closely with federal agencies, county extension agents, conservancy districts, and local health officials in the project area. GROUND WATER MONITORING PROGRAM The monitoring program presented in this report focused on groundwater quality monitoring in one of Colorado's major agricultural regions, the Arkansas River Valley. A map of the study area and sample locations is provided in Figure I, The monitoring program included sample collection, laboratory analysis, and data analysis and storage. Upon completion of the full analysis, which will include integration with previous and current studies by other agencies, this sampling program will provide the basis for determining a groundwater quality baseline for this region. The Ag Chemicals Program of the Water Quality Control Division sampled one hundred thirty nine (139) domestic, stock, and irrigation wells throughout the valley (Figure I). This sampling program was the first effort to monitor the entire Arkansas River Valley alluvial aquifer to establish the possible impacts and magnitude of agricultural chemical contamination, This region is characterized by intense irrigation agriculture encompassing both surface water diversions and wells for irrigation water supplies. The wells supply surface and center-pivot irrigation systems from the shallow unconfined aquifer. In August 1995, a confirmation sampling program was conducted to confirm pesticide detections and elevated nitrate levels measured in 1994. Wells were selected for sampling based on a favorable location within the shallow alluvial aquifer, general well and site conditions, and cooperation of the well owner. The wells were sampled once between July and December, 1994 by Brad Austin and John Colbert ofCDPHE, Field sampling procedures followed the protocol developed by the ground water Quality Monitoring working group of the Colorado nonpoint task force. Well samples were analyzed for basic water quality constituents, dissolved metals, and selected pesticides. A list of analytes is presented in Table 1. The basic inorganic analysis was performed by the Soils Laboratory at CSU with all samples split with the CDPHE Laboratory for nitrate. Comparison of these split parameters shows consistent results between the two laboratories.