Presentation on theme: "WRD Research Project: Tracer Test to Track Recycled Water Movement at the Spreading Grounds."— Presentation transcript:
WRD Research Project: Tracer Test to Track Recycled Water Movement at the Spreading Grounds
Purpose of the Research Project: Determine how long it takes for groundwater to move from the spreading grounds to nearby wells. Simulate recycled water movement. Acknowledgements County of Los Angeles DPW South Montebello Irrigation Dist. City of Pico Rivera Cal Water Service Montebello Land & Water Pico Water District La Habra Heights County WD City of Whittier California DHS University of California at Santa Barbara – Cooperating Agency: Dr. Jordan Clark
Problem Definition Recycled Water has been used for groundwater recharge in the Montebello Forebay > 40 yrs. New DHS Draft Regulations will not allow drinking water wells within 500 feet of recharge ponds unless recycled water stays underground >6 months. 23 production wells exist within 500 feet and may be impacted by the new regulations at a cost of $40 million to replace these wells if needed.
Rio Hondo San Gabriel River 1 Mile Rio Hondo Spreading Grounds San Gabriel Spreading Grounds Sewage Treatment Plants W.N. Dam Whittier Pico Rivera Montebello
San Gabriel River Spreading Grounds Rio Hondo Spreading Grounds
Background High quality recycled water has been recharged at the spreading grounds since 1961. Over 1.2 million acre-feet have been recharged to date. Recycled water is a reliable, high quality replenishment supply that is available year round. The DHS has issued Draft Regs that may impact production wells near the spreading grounds.
DHS draft regulations (12/01/04) for using recycled water at spreading grounds: …recharge water shall be retained underground for a minimum of six months prior to extraction…and shall not be extracted within 500 feet of a point of recharge. But allows for an alternative... The project sponsor may…reduce the distance…if the sponsor can demonstrate with tracer test results…that the required retention time will be achieved…
Draft Regulations Spreading Grounds w/ Recycled Water Drinking Water Well Minimum 500 feet away 6-Months Recycled Water underground for more than 6 months before pumping out Aquifer
Geologic Effects on Travel Time Spreading Grounds w/ Recycled Water Drinking Water Well Aquitard Aquifer Weeks/ Months Months/ Years
How was the Research Done? Add a tracer to the spreading grounds to tag the water. Time how long it takes to reach nearby wells. Dr. Jordan Clark of UCSB added the tracer and performed the analyses. WRD staff sampled the wells. Tracer Selected: Sulfur Hexafluoride (SF6) gas. Inert, safe gas that moves with the groundwater. DHS-approved this tracer for this test.
Why Use a Tracer? Recycled water can not easily be identified in a groundwater sample. Therefore, a compound (tracer) must be added to act as a surrogate for the recycled water. Recycled water
How was the Research Done ? Identified available wells. 17 of the 23 production wells available. 1 background well. Also monitoring wells. Background testing to confirmed no tracer in the wells water prior to the test.
Tracer Test Started Feb. 2003 Heavy rains filled all basins. Tracer added to spreading grounds water and sampled to confirm its presence. Sampling of wells biweekly, monthly, and bimonthly. Sampled through February 2005 (2 year test).
2-Year Tracer Experiment Results Arrival Time is a Function of Depth Good Correlation
2-Year Tracer Experiment Results Arrival Time is a Poor Function of Distance Poor Correlation
Discussion of Results Tracer successfully transferred into aquifers. Only 4 of 17 production wells saw the tracer within 6 months. (Two of these are not within 500 feet of recycled water ponds). Depth and geology affect travel time more than horizontal distance. DHS agrees: Most wells can operate as is. Very good news as only two of 17 may need modifications or replacement.
South Montebello #5 Test Spreading Grounds w/ Recycled Water 200 219 399 148 60 feet 2 Months Travel Time > 6 Months Travel Time