Groundwater Quality
The best quality groundwater is usually derived closest to the geologic outcrop areas of each aquifer. Generally, the water quality of groundwater derived from each aquifer is variable and dependent upon a variety of factors including, but not necessarily limited to, distance from the recharge area, aquifer transmissivity and storage, groundwater flow rates, aquifer rock type, dissolution of soluble salts within the aquifer matrix, and leakage of poor quality water into the aquifer from adjacent units. Groundwater in Platte and Goshen Counties has historically exhibited high levels of some regulated constituents. Local hydrogeology and agricultural practices are contributing factors to lower groundwater quality in these areas, especially in the groundwater of the alluvial deposits along the North Platte River. In some areas, municipal well water contains elevated levels of natural radon gas, arsenic, and uranium, which are attributed to the local hydrogeology. In addition, agricultural practices have increased nitrate levels in the water supply wells of several communities and domestic wells within the Guernsey to State Line subbasin.

Groundwater Development
Agricultural wells are permitted for approximately 94 percent of total permitted groundwater used in the Guernsey to State Line subbasin and these wells commonly obtain water from the Quaternary and/or the Late Tertiary Aquifer Systems. Municipal wells are the second largest permitted user of groundwater in this area at about 4 percent of the total subbasin permitted use. Seven municipal and community public water systems are located within the subbasin. Industry is permitted to use approximately 2 percent of total permitted groundwater use, and sugar production is the primary industrial use. Industrial use also includes ethanol production. Domestic use wells are a very minor part of the total subbasin groundwater use. Due to the hydrologic connection between the North Platte River and the Quaternary Aquifer System in the Guernsey to State Line subbasin, the 2001 Modified Decree designated an irrigated area in Goshen County as the ôTriangleö. The Triangle is defined as the area bounded by Whalen Diversion Dam on the west, the Wyoming-Nebraska state line on the east, a line parallel to and offset by 300 feet south of the Gering/Fort Laramie Canal on the south, and a line parallel to and offset by one mile north of the Interstate Canal on the north. This area includes a significant number of high capacity irrigation wells that were constructed between the 1940Æs and 2004.

Prairie Center Control Area
On November 29, 1977, the State Board of Control held a public hearing to address the establishment of a groundwater control area in the northern Goshen and southern Niobrara Counties, which is located in the northeast corner of the Guernsey to State Line subbasin. The Prairie Center Control Area was established on December 2, 1977, by the State Board of Control in response to the declining groundwater levels in the Quaternary and Late Tertiary Aquifer Systems and potential conflicts between stock watering and irrigation wells. The portion of the Prairie Center Control Area located in the Guernsey to State Line subbasin can be seen by clicking here. Due to socio-political and hydrogeologic differences between the northern and southern parts of the Control Area, the Board of Control voted on January 31, 1978 to delete Niobrara County lands from the Control Area after the March 1979 elections of the Prairie Center Control Area Advisory Board (State Board of Control, 1978). The Prairie Center Control Area will play a role in the development of future high capacity wells within this area of the Guernsey Dam to Nebraska subbasin.