Introduction
The purpose of the groundwater monitoring program is to evaluate possible impacts to groundwater quality from current and past use of agricultural chemicals in Colorado.
The groundwater sampling program is intended to fulfill the following objectives:
- Determine if agricultural chemicals are present in the groundwater.
- Provide data to assist the Commissioner of Agriculture in the identification of potential agricultural management areas.
Groundwater samples have been collected by the program annually since 1992. Sampling events, which are designated by “Region” in this database, consist of regional and county screening, follow-up sampling of impacted areas, and a long term, annual monitoring network in the South Platte alluvial portion of Weld County.
Monitoring Area Selection
Factors initially considered by the program in choosing a study area for monitoring include:
- The monitoring area has a significant use of agricultural chemicals.
- The groundwater occurrence in the area is either in a major aquifer or a significant portion of groundwater within the area is shallow in depth.
- A significant portion of the area is irrigated.
- The soil types are conducive to leaching of agricultural chemicals.
- The aquifers are utilized for domestic water supplies in the area.
In general, based on the land use and hydrogeologic factors, an area is sampled if the potential exists for migration of agricultural chemicals into the groundwater. An additional consideration may be if the area is currently the subject of other scientific research into agricultural impacts to groundwater quality. Over time, study areas have been monitored as resources allowed that do not meet all of the categories listed above, but had local interest and cooperation in determining groundwater quality.
Well Selection
The program has sampled domestic, household, livestock, irrigation, municipal, and dedicated monitoring wells. During the first years of the monitoring program, wells were selected for sampling based on the following factors: permitted use, located within the target aquifer, or one of its major tributaries, and cooperation of the well owner. The wells were sampled once usually during the period June to August. The factors used in selecting pre-existing wells for this monitoring program included:
- Low flow wells permitted or used for domestic purposes first, stock wells second, and irrigation wells third.
- Completed within the target aquifer, or a connecting branch.
- Location generally down gradient with respect to agricultural activities.
- Depth to groundwater less than 150 feet, generally less than 50 feet.
- Well currently in use or at least has a working pump installed.
- Direction of groundwater flow.
- Wellhead and casing in apparent good physical condition.
- Wellhead area appears to be free of point sources of contamination.
- Well owner consent to participate in the monitoring program.
More recently, the groundwater sampling program has shifted its focus from sampling existing domestic, stock, and irrigation wells to dedicated monitoring wells. These wells, specifically installed for the purpose of collecting groundwater quality samples, provide a higher quality sample free from the pitfalls associated with other well types. Primarily, the potential for false positive results exists with many pre-existing wells because of sub-standard well construction or less than ideal sampling conditions. In the case of domestic wells, the location and condition of the associated septic system was usually unknown and the freedom of influence from a spill was dependent on the wells owners’ memory. Although additional considerations are generated by the switch in well type, it is believed a more verifiable representation of groundwater contamination, or the potential for contamination, in groundwater can be determined.
Dedicated monitoring wells have been sampled in the alluvial aquifers of the Lower South Platte basin, the San Luis Valley, Mesa County, the Arkansas Valley, and the Urban Front Range. The South Platte basin monitoring wells were established by the Lower South Platte Water Conservancy District and the CDPHE and the San Luis Valley wells were part of a network established by the US Geological Survey (USGS) NAWQA program. The Mesa County and urban Front Range wells were installed by a wide variety of groups. Additionally, the groundwater program installed 20 dedicated monitoring wells along the lower Arkansas River, from Pueblo to Holly Colorado in 2004.
Long-term Network
In 1995, a long-term monitoring effort was initiated in the South Platte alluvial aquifer from Brighton to Eaton. This important agricultural area had been identified with water quality problems from previous work and the program goal was to determine, if possible, the current trend in groundwater quality as well as effects of Best Management Practices currently being implemented in the area.
The long-term monitoring network consists of three sets of distinct well types: 20 dedicated monitoring wells operated by the Central Colorado Water Conservancy District (CCWCD) that have been sampled each year since 1995; 60 irrigation wells sampled in 1989, 1990, 1991, 1994, and each year since 1995; and 18 domestic wells sampled in 1992, 1995, and every third year since. In the database, these wells are designated Weld County Monitoring, Weld County Irrigation and Weld County Domestic, respectively. The monitoring wells are sampled in June and the irrigation and domestic wells in July and August of each year.
The monitoring wells typically sample the upper 10 feet of the aquifer. This region of the aquifer is typically first impacted by contaminate transporting recharge. The irrigation wells are typically screened through the entire saturated zone and thus provide an average water quality for the entire aquifer. Domestic wells typically pull their water from the bottom 20 feet or so of the aquifer.
