A ground water hydrology investigation was conducted in the vicinity of the City and Borough of Juneau (CBJ) well field in Last Chance Basin. This study was undertaken to evaluate the ground water conditions during Gold Creek winter low flow events. The results of this investigation are summarized below.
Gold Creek is a small, dynamic, high energy stream with an 8.5 square mile drainage area above the USGS stream gage at the head of Last Chance Basin. A portion of Gold Creek's flow is captured by old mine glory holes (sink holes created by the collapse of underground mine workings), routed through the mine, and redirected back to Gold Creek above Last Chance Basin. During low flow conditions, the entire flow of Gold Creek infiltrates into the ground water system within the basin.
In March 1994, between two and three hundred dead char were found in a pool on Gold Creek located near the bottom of Last Chance Basin. The death of these fish coincided with a turbidity event that was described as an ~unusual milky sediment discharge~ (Attorney General Report, 1995). The sediment event and dead fish raised public concern, and a series of public meetings were held to evaluate the possible causes of the fish kill. Numerous possible causes were identified, and this study helped evaluate low water flow in Gold Creek. Initially, low flow was ruled out because the condition of the fish suggested that the fish were not frozen, nor had they died from lack of oxygen; conditions that would be expected if the creek went dry. A second low flow event during this study provided key data for evaluating low flow conditions.
To better understand the unconfined aquifer's response to pumping from the well field and winter low flow conditions, the Department of Natural Resources conducted a joint study with the CBJ and Echo Bay Mines. The study consisted of five shallow drive point wells installed in Last Chance Basin and equipped with water level monitoring equipment. This presentation summarizes the information obtained from the study and discusses the results.
The water table varied directly with Gold Creek stage until Gold Creek flow decreased to approximately five cfs. At approximately five cfs, Gold Creek flow did not significantly change, but the water table continued to decrease, reflecting the greater CBJ pumping rate of seven cfs. Once Gold Creek flow increased, by as little as two cfs, the unconfined aquifer recharged within two days. The water table increased up to six feet in 24-48 hours.
During the January 1995 low flow condition, springs below the basin drained Last Chance Basin at a rate of 3.5 to 5.3 cfs. The springs act like a pumping well in which the cone of depression into Last Chance Basin. During low flow, the cone of depression dewaters the creek in an upstream direction from the basin outlet.
The January 1995 fish kill (during the study period) occurred simultaneously with a rapid decrease in the water table at the lower end of Last Chance Basin. The drop in the water table caused the lower portion of Gold Creek to become a loosing reach and go dry in a matter of hours. This rapid decrease was caused by the springs below the basin interacting with the drawdown influences from the CBJ well. The cycling of the pumps caused a rapid lowering of the water table. When the pumps cycle off, the accompanying increase in the water table provided base flow to previously dry reaches of the creek. It appears that fish have adapted to this type of event by burying themselves into the moist sediment to wait for the water table to increase. Layers of ice and snow over the dewatered pond prevent the fish from freezing or drying out.
Rick Noll DNR-DMWM 400 Willoughby Ave. Juneau AK 99801 Phone: 907/465-3487 Fax: 907/586-2954 E-mail: rickn@dnr.state.ak.us