ELECTRICAL RESISTIVITY IMAGING OF THE TAMARACK RANCH STATE WILDLIFE AREA, NORTHEASTERN COLORADO

The Tamarack Flow Augmentation Project located along the lower South Platte River in northeastern Colorado was initiated to augment in-steam river flow during low flow months in endangered species habitats in Nebraska. Water is pumped from wells in the alluvial aquifer near the river to recharge ponds located upland. Previous studies based on sparse drilling data identified a possible paleo-channel of the South Platte River beneath the recharge ponds that may influence recharge groundwater flow. To better delineate the subsurface stratigraphy, a seven line DC resistivity survey oriented perpendicular to the suspected location of the channel was used. The geophysical data, correlated to boring logs, delineated three lithostratigraphic units with distinctive geophysical characteristics. From the surface downward these include 1) an aeolian sand layer of moderate resistivity (600-300 ohm-m) up to 30-m thick within the survey area; 2) an approximately 30m thick moderately-low resistivity (200-80 ohm-m) poorly sorted layer, interpreted as alluvial material; and 3) a low resistivity (50-10 ohm-m) bedrock layer, approximately 60m in depth, interpreted as the Brule Shale. The water table correlates with the base of the aeolian sand layer indicating that the alluvium is the primary aquifer. One survey line imaged the contact between the alluvial layer and the Brule layer at the northern edge of the paleo-channel, but the other lines showed no evidence of a channel. There was a strong correlation between the boring logs and electrical resistivity images, indicating that resistivity is an effective method for imaging the top and bottom layers of the aquifer in this area.