MALIBU WATER QUALITY
case, the bedrock elevation was found to be less than a meter from the predicted levels based on the surface geophysics. Further, the drilling confirmed that the basin is comprised of up to six meters of alluvium, three to six meters of clay or clayey silt (LPZ), and 27.4 to 33.5 meters of CCGs.
Several aquifer tests were conducted on the CCG aquifer during the study. These resulted in moderate to high transmis- sivities of 372 to 620 meters squared per day (m2/day). The specific capacities ranged from 2.3 to 5 liters per second per meter (lps/m) (RMC, 2013). Both of these values were deemed good for potential treated wastewater injection and is within the range expected for gravelly sands and an aquifer that is approximately 30.5 meter thick.
Water quality monitoring conducted in 2013 in wells screened in the lower CCG were combined with electrical resistivity survey data to identify the location of the fresh water-salt water interface. Based on these results, the presence of a fresh groundwater lens offshore indicates that convective flow of groundwater is disrupting the saline water interface that would be predicted by the Ghyben-Herzburg (Kim and Chon, 2007) relationship used to measure the thickness of a fresh water lens under conditions of no groundwater flow. The fresh-to-brackish water zone appears to rise towards the sea floor, south of the beach, suggesting that groundwater is discharging out through the sea floor. Discharge is occurring as far as 91 meters offshore, either due to stratigraphic controls or due to buoyancy effects of a freshwater lens lying over the more saline water (Cardno-Entrix, 2013).
Water budget
Sources of groundwater recharge to (inflow) and discharge from (outflow) the alluvial deposits in the basin, as described by McDonald Morrissey, occur by several different processes (McDonald Morrissey, 2014).
A water budget was proposed based on the sources of inflow and outflow discharge. The values were specified by McDonald Morrissey, (2014) and were based on an overall balanced water budget (Table 1). This was used as the basis for the groundwater modeling as described below. The larg- est sources of recharge were, as expected, stream infiltration (Malibu Creek), making up approximately 44%, and wastewa- ter, within the alluvium and upland comprising nearly 36% of the basin recharge. The groundwater outflow was dominated by the ocean outfall at 71% and the lagoon representing 22% (McDonald Morrissey, 2014).
Sinks (discharge and/or outflow) for groundwater were found to be slightly less than that of the recharge; however, the variation is slight and well within the margin of error. There are presently no pumping production or remediation wells within the study area.
Groundwater Modeling
Several iterations of groundwater modeling have been con- tracted by the City of Malibu over the past decade. All of these model iterations were performed by McDonald Morrissey, (2012 and 2014). The last iteration was done for the purpose of determining the impacts of treated wastewater injection and reuse within the MCGB. This included determination of the optimal location for treated wastewater injection within the basin and the rate the aquifer system could sustain without negative impacts to existing shallow septic systems and foun- dations. It was predetermined that a possible impact would be the rise in water levels within the shallow unconfined aquifer due to the wastewater injection. Another potential negative
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impact was that of the injected wastewater on Malibu Creek and Lagoon. The modeling effort was also used to help under- stand where the highly treated wastewater that is injected into the lower CCGs would ultimately flow. These constraints and conditions were used within the groundwater flow model to develop a wastewater collection, treatment, and disposal plan for the entire CCAM.
Based upon the modeling efforts it was determined that a maximum of 2.3 million liters per day of treated wastewater could be safely injected into the CCAM without any unaccept- able groundwater level increases within the shallow uncon- fined aquifer above the LPZ. Additionally, it was determined that as much as 400,000 liters per day of treated wastewater could be percolated through a series of seepage pits within Winter Canyon to the west of the CCAM. This Winter Canyon groundwater flow was consistent with previous modeling efforts conducted by Earth Consultants in 2000. In each modeling scenario, the CCAM injected treated wastewater discharged to the ocean and not towards Malibu Creek or the Lagoon (McDonald Morrissey, 2012 and 2014; RMC, 2012).
Conclusion
Based upon the work completed, it was determined that the best opportunity to dispose of the excess treated waste- water was to inject the water into three wells (IV-1, IV-2 and IV-3) along Malibu Road (Fig. 3), screened through the CCG. Between the aquifer testing and modeling, it was determined that a single well would work for the City’s needs; however, three injection wells were chosen as this would allow for backup wells while maintenance was occurring. In order to reduce the overall amount of treated wastewater needed for injection, each property that sent wastewater to the City for treatment, would be required to reuse as much treated wastewater as they could, through a “purple pipe” system. This highly treated water could be used for irrigation; non-potable uses such as toilets and for decorative ponds. The usage of this water will vary throughout the year, with summer months having a higher demand for reuse water than winter months.
The first phase of the treatment and disposal system went online in 2017. During this first Phase, less than 750,000 liters per day went through the City’s advanced wastewater system. However, the City of Malibu is planning on implementing Phase II sometime in 2024.
With the replacement of traditional septic systems by a municipal system, such as the City of Malibu has implemented, overall water quality both in the shallow groundwater and nearshore areas should improve. It took over 30 years, but finally the State of California and the City of Malibu were able to come to an agreement that helped protect Malibu Creek, lagoon, and nearshore area.
AIPG 2021 National Meeting Call for Abstracts/ Student Posters
Information and online submissions:
www.aipg.org/page/2021CACallforAbstracts Deadline: June 7, 2021
Selection Notification: June 21, 2021 Technical Sessions: October 25-26, 2021
Apr.May.Jun 2021 • TPG 19
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