Lysimeters assess regulatory compliance with recycled water recharge

Joe LeClaire

Water agencies in Southern California use lysimeters to assess regulatory compliance with recycled water recharge into a potable groundwater basin.

Rapid urban growth in Southern California leaves the region’s water agencies no choice but to elevate the supply of safe and reliable water as the top priority issue facing the region.

The Southern California Association of Governments (SCAG) reports that Southern California’s population increased by more than 12 percent from 1990 to 2000 - an influx of almost two million people. While Southern California’s overall growth is impressive, the population of Riverside and San Bernardino Counties - collectively known as the “Inland Empire” - grew by an astonishing 26 percent!

The Inland Empire Utilities Agency (IEUA), the Chino Basin Watermaster (Watermaster), the Chino Basin Water Conservation District, and the San Bernardino County Flood District are working together to expand and improve 19 recharge basins to enhance the agencies’ ability to capture and recharge stormwater, recycled water, and imported water. These sources of water supply will be recharged into the Chino groundwater basin and will ensure the stakeholders that the region is drought-proofed to the greatest extent possible. Richard Atwater, the chief executive officer and general manager of IEUA stated, “IEUA will continue to strive to make its service area less dependent on imported water. By working together with our local retail water agencies, we will ensure that our communities continue to have high quality, reliable water supplies with very competitive water and sewer rates.”

IEUA has spent about US$ 75 million dollars to date and is committed to spending another US$ 100 million dollars over the next three years for recharge basin enhancement and expanded recycled water projects. These projects will produce 95,000 acre-feet of new water annually, enough to meet the needs of 300,000 new residents.

Meanwhile, the Watermaster is actively implementing the Basin’s Optimum Basin Management Program (OBMP), which includes extensive monitoring, further developing recharge capabilities, storage and recovery projects, managing salt loads, developing new yield such as reclaimed and stormwater recharge, and continuing to work with other agencies and entities to enhance this significant natural resource. Ken Manning, the chief executive officer of the Chino Basin Watermaster said that “a centerpiece of the OBMP is enhancement of the basin recharge capacity, so that high quality storm water and available recycled water can be retained in the Chino groundwater basin.”

Basin Plan

Recharging recycled water into a potable groundwater basin, required that the current Basin Plan had to be first reviewed, evaluated, and ultimately amended. Secondly, the groundwater quality objectives for the basin had to be raised through a rigorous methodology that was consistent with state law.

The Santa Ana Water Quality Control Plan (Basin Plan) that was in force when the project was being considered was last modified in 1995. The Basin Plan is adopted and administered by the Regional Water Quality Control Board, Santa Ana Region (Regional Board). The Basin Plan is the basis for the Regional Board’s regulatory programs and establishes water quality standards for all groundwater and surface waters of the region.

The Porter-Cologne Water Quality Control Act (Division 7 of the California Water Code), and the Federal Clean Water Act both mandate periodic review of these basin plans. Section 13240 of the Water Code requires, "Each regional board shall formulate and adopt water quality control plans for all areas within the region… Such plans shall be periodically reviewed and may be revised."

During consideration of adoption of the updated Basin Plan, watershed stakeholders questioned the validity of the groundwater quality objectives for total dissolved solids (TDS) and nitrate nitrogen and the Regional Board’s Nitrogen/TDS management plan that implemented those objectives. A principal underlying concern was that the updated basin plan constrained inappropriately wastewater recycling opportunities. Since the reuse of recycled water is a critical component of many agencies’ plans to meeting rapidly increasing water demands in the region, the Regional Board agreed to make the review of the objectives a high triennial review priority.

The Nitrogen/TDS Task Force (Task Force) was formed in 1995-96 to conduct studies regarding the TDS and nitrate-nitrogen objectives and other components of the N/TDS management plan. The Santa Ana Watershed Project Authority coordinated the task force, which is comprised of 22 water supply and wastewater agencies throughout the region. Regional Board staff members were active participants in this effort. Findings and recommendations based on the Task Force studies were presented to the Regional Board at numerous public workshops during the course of the studies.

Current law and regulations guided the Task Force studies. The Task Force recommendations for changes to the TDS and nitrate-nitrogen water quality objectives for groundwater within the region are based on consideration of factors specified in Water Code Section 13241 and the state’s antidegradation policy (SWRCB Resolution No. 68-16). The economic implications of all recommended changes to the N/TDS management plan were also considered.

In certain groundwater basins in the watershed, the TDS and nitrate objectives were raised to allow the recharge of recycled water. IEUA had to demonstrate to the regional board that raising the objectives would be consistent with the maximum benefit to Californians, would not unreasonably affect present and anticipated beneficial use of such water, and would not result in water quality less than that prescribed in the basin plan amendments.

These amendments will ensure the reasonable protection of the beneficial uses of surface and groundwaters within the region and are consistent with the state’s antidegradation policy (SWRCB Resolution No. 68-16).

On 8 September 2004, the State Water Resources Control Board approved the basin plan amendment, which was subsequently approved by the Office of Administrative Law and US Environmental Protection Agency.

Lysimeters monitor compliance

Two California state regulatory agencies -- the Regional Board, and the State Department of Health Services (DHS) - provide oversight of the Chino Basin Recycled Water Groundwater Recharge Program. The DHS has been developing draft regulations for the recharge of recycled water. Early versions of these proposed regulations required the project proponent to drill and install monitoring wells in the recharge “mound” beneath the recharge basin to demonstrate compliance - reduction of total organic carbon and total nitrogen to concentrations specified in the permit. A groundwater mound is the local rise of the water table above its natural level resulting from a localized source, such as a recharge basin.

Representatives from the US consultancy Wildermuth Environmental, Inc. (WEI) and Watermaster were quick to point out that drilling such a “mound-monitoring well” may not be feasible in the Inland Empire. Depth to the water table is typically greater than 300 feet and the presence of interbedded lenses of fine-grained material stops the water infiltrating from the recharge basin from traveling completely vertically to the aquifer. Furthermore, the water table can fluctuate by more than 20 feet over several years, so it would be necessary to drill and install mutipiezometer or nested wells, which would increase project costs. Finally, one could not be assured that the well would provide samples representative of only the recharge mound.

Instead, WEI recommended that the DHS consider permitting the use of lysimeters to monitor compliance with the recharge permit. A lysimeter consists of a porous cup and reservoir installed in unsaturated soil that collects undiluted samples of recharge water prior to reaching the regional groundwater table. WEI installed these lysimeters directly in the bottom of the recharge basins to ensure that samples collected while the recharge basin is being operated are truly representative of the recharged water.

The lysimeter also has two tubes that run up the casing to the lysimeter head assembly. Conduits route the tubes to lysimeter head assemblies that were constructed on the top of the recharge levee.

Lysimeter sample collection procedures are straightforward. Upon installation below ground level, a vacuum is applied to the lysimeter through the tubing leading from the lysimeter to the ground surface. The negative air pressure created inside the lysimeter draws pore water into the lysimeter through the porous, stainless steel section of the lysimeter. The pore water is brought to the surface by applying positive pressure to the lysimeter through a second tube. At the surface, the pore water is collected in a collection bottle.

After months of biweekly sampling, IEUA and Watermaster found a consistent 70 to 80 percent reduction in TOC concentrations from the recycled water at the surface to the deepest (25 feet) lysimeter due to processes related to soil-aquifer treatment (SAT). Total nitrogen concentrations from IEUA’s advanced treatment plants are extremely low and already meet permit requirements.

The nexus of the OBMP, which mandated the enhancement of Chino Basin’s recharge capacity, and the Basin Plan amendment, which allowed for raising the groundwater objectives to allow for the recharge of recycled water, will ensure the supply of high quality and reliable water for the people in the region, according to Mark Wildermuth, the chief executive officer of Wildermuth Environmental, Inc. “Moreover, this project illustrates the way in which water agencies and regulatory agencies are effectively working closely together for the betterment of the people of California,” he added.

Author’s Note

Joe LeClaire is the chief operating officer of Wildermuth Environmental Inc., based in Lake Forest, California, USA. For more information, visit the website www.wildermuthenvironmental.com.