ASR Metric = (T*maxDh)/(0.183*Q(log T + 15.3))
Where T = transmissivity, max Dh = depth to water, Q = injection rate. Conversely, a result less than one suggests the combination of transmissivity, depth to water and the target injection rate is not compatible; potential problems may include an undesirably large increase in water levels or pressures leading to new surface seeps or an insufficient maximum storage capacity. However, these hydraulic factors have a flexible relationship. High transmissivity can compensate for small depth to groundwater and vice versa. The site rating index evaluates several aquifer parameters separately and produces a percentage of ideal conditions for managed underground storage.
In Oregon, 10 out of 12 existing underground storage projects target fractured basalt interflow zones, where average transmissivity is greater than 1,500 m2/d (16,200 ft2/d) and native groundwater quality is generally suitable for drinking water. These units are also prone to groundwater decline due to over-pumping, which essentially creates storage for ASR. Surface spreading recharge operations target glacial flood deposits and other surficial unconsolidated aquifers. Results show that suitable potential sites correspond predominantly with Columbia River Basalt, Quaternary and late Tertiary sediments. While current annual managed underground storage totals about 3.7 x 107 m3 (9 billion gallons or 30,000 ac-ft), potential aquifer storage is approximately equal to Oregon's annual public water supply, or 8 x 108 m3 (220 billion gallons or 670,000 ac-ft). This suggests that underground storage expansion in Oregon can provide significant flexibility in the timing of water availability.