Tuesday, April 1, 2008 : 11:00 a.m.
Mapping Sea Water Intrusion using Time Domain Electromagnetic Induction and High Resolution Electrical Resistivity, Southern Los Angeles County, California
The extent of the intrusion is complicated due to concentration and density differences of the saline water, the gradients and hydraulic properties of the sandy aquifers, and the shape and width of the fresh water/salt water interface. Previous mapping of the intruded sea water (also known as the saline plume) has relied upon water quality samples from monitoring or production wells. However, these point data bias the intrepretation because of the limited number of wells available and the depths at which they are screened may miss or average the true salinity in the aquifer at that location.
In 2006, the Water Replenishment District of Southern California commissioned a geophysical survey to pilot test various methods of mapping brackish water in a heavily urbanized setting. Both Time Domain Electromagnetic Induction (TDEM) and High Resolution Electric Resistivity (ER) soundings were employed in available open spaces such as parks, baseball fields, and drainage channels. The ER soundings provided high resolution images of the upper 250 feet, but could not reach the targeted depths of 400 to 800 feet due to the lack of an open corridor long enough for the survey. The TDEM method was used with customized multiple turn transmitter loops of 40m x 40m or smaller to minimize cultural interference and reach the targeted depths. The brackish water was successfully mapped at these locations with results comparing favorably to nearby wells. Due to the success, a full scale TDEM survey is planned for early 2008.
Theodore A. Johnson, PG, CHG, Water Replenishment District of Southern California Ted Johnson is the Chief Hydrogeologist at the Water Replenishment District of Southern California. He leads the Basin Management and Water Quality Department, providing technical analysis, review, and oversight for projects related to artificial recharge, seawater intrusion, groundwater quality, conjunctive use, computer modeling, recycled water, tracer tests, and groundwater production. Ted received his B.S. and M.S. degrees from the California State University at Fullerton and is a California Professional Geologist and Certified Hydrogeologist with over 20 years of experience in Southern California groundwater investigations.
John Jansen, Ph.D., Aquifer Science & Technology Dr. Jansen has over 20 years of experience in ground-water resource development with respect to environmental and engineering geophysics. Dr jansen has conducted over 200 ground-water resource investigation throughout the United States and Mexico. He is the co-author of the Borehole Geophysics chapter in the 3rd Edition of Ground Water and Wells.
Nancy Matsumoto, PG, CHG, Water Replenishment District of Southern California Nancy L. Matsumoto, P.G., C.HG. is a Senior Hydrogeologist at the Water Replenishment District of Southern California. She holds a B.S. in Geology from Texas A&M University, and a M.S. in Geology from California State University – Los Angeles. Ms. Matsumoto has over seventeen years of professional experience in the water resources management, water resources consulting and environmental consulting fields.
Bennett Chong, Water Replenishment District of Southern California Benny is an Associate Hydrogeologist responsible for field activities such as groundwater sampling, water level collection, and data logger deployment and downloads. Office responsibilities include data analysis, reporting, sampling coordination, database maintenance, and project management.