Loading...
HomeMy WebLinkAbout2022-09-10 review of disposal concept (CHA) Cleath-Harris Geologists, Inc. 75 Zaca Lane, Suite 110 San Luis Obispo, CA 93401 (805) 543-1413 Disposal concept review 1 9/10/2022 September 10, 2022 Dan Garson Wathen Castanos Homes 735 Tank Farm Road, Suite 100 San Luis Obispo, CA 93401 Subject: Proposed Disposal of Groundwater Inflow to Basin, Avila Ranch Project Dear Mr. Garson: Cleath-Harris Geologists has reviewed the proposed conceptual design for a basin drain to dispose of groundwater that has intruded into the storm drains for the Avila Ranch project , and then flowed into the basin (RRM Design Group, 8/4/2022). The basin was sized to receive storm flows and project site surface runoff, and was expected to be dry otherwise. The flow of intruded groundwater has resulted in significant standing water in Basin A. Cleath-Harris Geologists’ review pertains to the source of the groundwater and the possible means of either preventing the intrusion into the storm drains or disposing of the water once it reaches Basin A. Shallow Groundwater Conditions Inflow to the storm drain system for the Avila Ranch project area has been observed where the storm drains are at or below an elevation of about 98 feet (RRM, August 2 , 2022). Some variation in groundwater level occurs over the year , which will affect the amount of flow intruding into the storm drain. This discharge from the groundwater is similar to what happens in the stream channel. Under wet conditions, the groundwater level rises above the stream channel invert and results in a gaining stream in the stream channel coursing through the property. The stream channel bed drops from an elevation of a bout 110 feet at the northern project property line to about 92 feet at the intersection of Buckley and Vachell. Sea sonally, the groundwater can be above the invert of the stream channel during wet periods and below the invert during the dry periods. When CHG staff walked the creek during our well siting investigation in June 2022, there was no flow in the creek. A pool of water was observed near the creek crossing (elevation about 102 feet). This August 23rd, the water level was at about 2 feet below stream channel invert a short distance upstream of the observed pool (102 foot elevation). Ground water level elevations are higher to the northeast on the Avila Ranch property. In the irrigation well on the project area, the groundwater level was measured at 5.5 feet below the top of casing, or at an elevation of about 99 feet (Cleath-Harris Geologists, June 7, 2021). Disposal concept review 2 9/10/2022 These existing conditions will result in a varying amount of groundwater flowing into the storm drains, depending on the season. Proposed Disposal Concept The groundwater flow in the storm drains that drain to the basin has been measured at about 30 gallons per minute and has a very low turbidity. The proposed drain would pump this water out of the basin and into an adjacent area that is designated as a wetland. The Basin A floor at the lower end of th e basin is at 93 feet elevation, with a subgrade elevation of 89 feet. The adjacent wetland area is at an elevation of 96 to 97 feet. The adjacent stream bed at the bridge is about 93 feet elevation. Alternative Options Alternative options that would lower the shallow groundwater and thereby eliminate flow into the storm drains were reviewed by the project team. One of these options was a gravity flow drain in the proximity of the storm drains (RRM, August 23, 2022). This option did not have enough head to result in lowering water lev els below the storm drain. Therefore, it would not be effective in diverting the groundwater prior to it entering the storm drain system. Another option would be to install dewatering wells at various locations. This option was not feasible due to the large area that would need to be dewater ed. On- and off-site delivery of the infiltrating groundwater in the storm drains to adjacent agricultural fields would be effective. It would be a beneficial use and could off-set existing or future irrigation demands and reduce groundwater production from wells. The demand for the water may be variable and not be able to handle the continuous flow without storage. A neighboring property has plans to put in an off-stream reservoir that could receive the water potentially for the duration of the discharge. Thi s design concept is something that could be explored but is not within the control of the project team. Design Considerations The flow rate for the final design of the proposed disposal concept should take into consideration the following groundwater conditions:  Flow rate variations from the storm drain outlets  Shallow groundwater that may flow directly into sides and bottom of the basin The groundwater inflow to the basin could possibly be as high as 100 ga llons per minute. Disposal concept review 3 9/10/2022 Conclusion Of the options available to remove the infiltrated groundwater that flows into Basin A, a pumped sump at the western end of the basin that discharges to the area adjacent to Buckley/Vachell intersection would be the most effective option at removing the water from the basin. Future use for irrigation of agriculture may be an option in the future but will require additional infrastructure and agreements with other parties. Respectfully Submitted, Timothy S. Cleath, Certified Hydrogeologist #81 Vice President, Cleath-Harris Geologist Disposal concept review 4 9/10/2022 References Cleath-Harris Geologists, June 7, 2021, Pumping Test Results of the Irrigation Well at Avila Ranch Cleath-Harris Geologists, August, 25, 2022, Findings from Passive Seismic Analysis at the Avila Ranch Development RRM Design Group, August 2, 2022, Avila Ranch Basin Modification to Basin A RRM Design Group, August 23, 2022, Potholing results to evaluate gravity dewatering drain.