DELINEATION OF MANAGEMENT ZONES FOR SOURCE WATER PROTECTION OF PUBLIC WATER SUPPLIES, HELENA VALLEY, MONTANA

Helena represents a significant urban area in Montana. The Helena Valley Source Water Protection Project represents a combined effort to assess water conditions for public water supplies (PWS) in the area following the guidelines of the Montana Source Water Protection Program. A three-year time of travel distance was calculated to delineate inventory zones for approximately 54 public water supplies in the study area. The valley is located in an intermontane basin covering approximately 150 square miles in the Northern Rockies. The valley is bounded by middle Proterozoic metasedimentary rocks to the north and east, primarily argillite and quartzite; early Paleozoic dolomite and limestone to the southwest; Cretaceous plutonic rocks to the west; and upper Cretaceous andesites and tuffs to the southwest. The valley-fill aquifer comprises up to 100 feet of Quaternary alluvium and Tertiary Pediments overlying up to several thousand feet of semi-consolidated Tertiary basin fill material thickest on the eastern side of the valley. All of the valley fill material comprises the unconfined Helena Valley Aquifer. Shallow ground water flows towards the lowest part of the valley, Lake Helena, which serves as a discharge point for the valley aquifer east towards the Missouri River. Time of travel calculations were determined using the Uniform Flow Equation. Aquifer properties were estimated by averaging values from local wells to ensure that results were consistent across the study area. Hydraulic Conductivity values for alluvium were estimated to range from 150 feet/day in the central part of the valley to 500 feet/day along the southern boundary. The hydraulic conductivity value for the Tertiary sediments was estimated at 60 feet/day across the valley. PWS Wells located along the margins of the valley are typically installed through shallow alluvium into deeper Tertiary sediments. Management zones for systems near the aquifer boundaries were delineated by determining time of travel distances separately for both Tertiary and Quaternary strata. The inventory zone reflects a combination of flow parameters for both parts of the aquifer. The delineated inventory zones for all PWS are managed using GIS, allowing for effective development of a comprehensive Source Water Protection Program for the valley.