Title
COWLITZ RIVER COLLECTOR WELL PHASE I INVESTIGATION RESULTS
COUNCIL INITIATIVE ADDRESSED:
Provide sustainable water quality & environmental infrastructure
CITY ATTORNEY REVIEW: N/A
SUMMARY STATEMENT:
In January of this year, exploratory drilling was conducted at three sites along the Cowlitz River to investigate the potential for developing horizontal collector wells as a source of water to replace Mint Farm groundwater. The results of the initial investigation were reported in March and prompted consideration of a fourth site, Rocky Point, nearer to the City’s water treatment facilities. In late April, staff was directed to conduct additional Phase I drilling and testing at Rocky Point.
Investigative work at Rocky Point was conducted in a manner identical to the other three sites. An 8-inch diameter test well was drilled and the subsurface geology logged to confirm the water bearing strata was sufficiently thick to support municipal demand. Following well development, the test well was pumped at low flowrates stepped up over a four hour period while continuously monitoring water level decline, i.e. drawdown.
Water quality samples were collected and analyzed for the same constituents analyzed at the other test sites. Test results show high levels of iron, manganese, hardness and silica, all characteristic of groundwater, and at levels comparable to or higher than Mint Farm groundwater. Water quality may change with longer and higher volume pumping, but the degree and rate of change is unknown. If iron levels at Rocky Point do not improve during extended pump testing, the highest level of iron treatment will be required.
Based on estimated yield potential, Rocky Point outperformed the other sites with a capacity of 9 to 11 MGD per collector well, and the ability to support two collector wells. Riverside Park also remains viable with the next highest estimated yield at 5 to 9 MGD, with the potential for multiple collector wells. Both sites...
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