MANAGEMENT STRATEGIES FOR RIPARIAN WATER SUPPLY WELLS

David Soong, George Roadcap, Yanqing Lian, and Chittaranjan Ray 

Illinois State Water Survey and University of Hawaii at Manoa
 

Abstract

Driven by the nation’s economic and population growth, demands for fresh water have increased and resource managers are compelled to evaluate alternative resources. Many communities, including some in Illinois, have used or planned to use alluvial aquifers as sources of water. The alluvial aquifers are connected to the nearby river systems; when the aquifer pumpage rate is higher, the potential to induce river water is higher. As a matter of fact, due to the limited capacity of most of the alluvial aquifers, many riparian wells are designed to pump induced river water by placement close to the river. In addition, the water quality of riparian wells is generally better than the quality of river water. Therefore riparian wells are plausible water sources for small or large communities where deep-rock aquifer or reservoir sites are not available. For small communities, since water from riparian wells is considered to be groundwater, in general it does not have to go through the same extensive treatments as those specified for surface water. However, studies have shown that the detentions of contaminants in riparian wells generally correlate with the timing of floods and contaminant concentrations in the nearby rivers.

In Illinois, approximately 13 to 15 towns have constructed wells in the riparian zones along the Illinois River. Higher pumpage increases the rates of induced infiltration of surface water; higher river stages or erosion at the channel bed also increases the possibility of induced infiltration. If contamination is related to natural water content, such as hardness, it can be detected and treated. The consequence is higher treatment costs. For other contamination such as pesticides and/or dissolved nutrients that vary with seasons, water quality is still a concern for well managers.

Since the impact on water quality in these alluvial aquifers comes mainly from the river under various pumping and recharge scenarios, preventive measures can be taken by conducting a thorough investigation of the movement of mass (river water) in the alluvial aquifer, i.e., river-to-well systems. Although our current knowledge about groundwater contamination by pesticides is lacking, these preventive methods will be conservative ones because pesticide transport follows the movements of groundwater.

The authors are developing rules to describe the relationships of these bank infiltration scenarios with the pump operations. A three-dimensional groundwater model has been developed for the Henry, Ill., site and we are conducting sensitivity analyses on physical factors that govern the water movement in the alluvial aquifer­river­well system. The results will be converted to operational rules for safe yield under a set of hydrogeologic conditions, and will be presented in nomographs (engineering charts). We will also compile current knowledge about pesticide contamination and the fate of pesticides in bank areas, and provide such information to aquifer users in a report form. Because the project has been reduced from the originally proposed two years to one year, we have had to adjust the scope of work in order to produce credible products. For the future scope of work, we will conduct risk analysis on the riparian wells using the Henry site as an example. The results will provide clearer information for managers/users using the developed nomographs.