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Mixed Vertical and Angle Well RBF Analysis
HDR EngineeringThe Lewis and Clark Rural Water System is developing a regional water supply in Clay County, South Dakota to serve communities in South Dakota, Iowa, and Minnesota. Water will be pumped from wells at several sites along the Missouri River south of Vermillion. Construction of three angle wells and one vertical well were completed in 2006. WHPA was contracted to oversee well testing, analyze the test results, and predict operational aspects of the completed wellfield.
The wells use riverbank filtration (RBF) from the Missouri River to provide raw water to the system. Riverbank filtration relies on the ability of a well to induce recharge from a nearby surface water source, so the degree of hydraulic connection between surface waters and groundwater is often the factor that limits wellfield capacity. A methodology for assessing the surface water/groundwater interchange, including the ability to assess horizontal or angled well screens, was needed. Furthermore, since the wells are located very near surface waters or existing high-capacity wells, data from the aquifer test can be affected by changes in river stages, pumping at neighboring wells, or even by unplanned interruptions to pumping. Removing these effects in traditional aquifer test analysis is time-consuming and inexact.
We integrated existing information and new field data to formulate an understanding of the aquifer characteristics and to build a conceptual model of the aquifer. Based on the conceptual model, we incorporated the field data into a regional groundwater flow model using a custom code developed by WHPA. The code integrates an analytic element model called wigaem, Fast Fourier Transforms, and inverse modeling with parameter estimation. The code requires a similar amount of data entry as a traditional code, but it specifically accounts for boundary conditions including streambank infiltration. We then used the groundwater flow model to evaluate the combined effects of pumping all of the wells.
The geologic data and aquifer test information, together with our cutting edge modeling techniques, allowed us to demonstrate that the aquifer is heterogeneous in the wellfield area and this heterogeneity impacts wellfield performance. Some of the wells are located in lower transmissivity settings and some of the wells are less connected to the river. We were able to tell our client where to invest to maximize yield. Additional recommendations were made about potential future locations for collector wells in this area.
Project Reference:
Dan Graber • Vice President • HDR Engineering
6300 S Old Village Place • Suite 100 • Souix Falls, SD 57108 • (605) 977-7767 • dgraber@hdrinc.com
