Thurston County Municipalities
Project: Feasibility Assessment for Recharge of
Location: Thurston County, Washington
Thurston County and the Cities of Lacey, Olympia, and Tumwater wanted
to expand their sewage treatment facility substantially. They explored
various discharge alternatives, including recharging large volumes of
treated wastewater to local groundwater systems using infiltration basins
and injection wells. This required a feasibility assessment to identify
suitable recharge sites and approaches.
In addition to identifying sites with high recharge capacities, we needed to consider many water-quality issues in our assessment. At the injection wellhead, water quality issues were a concern because chemical reactions could cause incrustation buildup on the screens, decreasing the well's capacity. Water quality impacts could also potentially affect the local aquifer system and existing users.
To identify possible sites for injecting or infiltrating groundwater,
we characterized various parameters throughout the county: soil types,
land uses, water-level depths, subsurface geology, locations of water
supply wells, and wellhead protection areas. A series of maps showed the
results of these analyses and provided a basis for selecting possible
recharge sites. We also reviewed literature on operational facilities
to identify treatment approaches that would meet regulatory standards
for water quality. Finally, we evaluated potential loading rates and soil
treatment processes for recharge basins and calculated land requirements
for a facility that could process 20 million gallons of treated water
The results of our feasibility assessment provided the basis for developing a new reclamation facility in the Hawkes Prairie area. We not only identified favorable sites for recharging reclaimed water, but we also noted key issues that would affect the ultimate design of a new facility.