ABSTRACT

1. HYDROGEOLOGIC CHARACTERIZATION AND ASSESSMENT OF IMPACTS OF WASTEWATER DISCHARGE ON THE BROAD BEACHWater Table Otto Figueroa, CSUF Geology, Graduate Student. W. Richard Laton, PhD, CSUF Geology, Assistant Professor. John H. Foster, PhD, CSUF Geology, Department Chair 6 ABSTRACT Investigations on the surface-subsurface water interaction play an important role in the development plans for housing project on coastal bluffs in the City of Malibu, California. A citywide sewage system does not exist in the City of Malibu; therefore all properties must use septic tank systems to dispose of wastewater. To insure the safety of beachfront homes, developers must assess the impact that wastewater discharged into the subsurface will have not only on their property, but also on the properties down gradient. A fourteen-acre property located on a bluff above the Broad Beach community is being considered for single-family home development. Current proposals estimate approximately 13,000 gallons of wastewater per day discharge into the subsurface for the development. To understand the wastewater-groundwater interaction at both the Trancas Canyon site and the Broad Beach community, a hydrogeologic investigation was conducted. From geologic data collected during well installation activities and from previous investigations, a structural model of the subsurface geology was developed. The authors interpret from the geologic model that an ancestral stream (Trancas Canyon Creek) cut a deep channel along the eastern portion of the property. Perched groundwater in the weathered zone of the Trancas Formation is present in the western portion of the property. On the eastern portion of the property, a saturated older alluvial aquifer is encounter at relatively greater depths. The well in the eastern portion appears to be tidally influenced, while the wells in the western portion are not. Since the geology and water levels are so disparate between the western and eastern portion of the property, a geometric solution for groundwater flow direction based solely on groundwater level data was not developed. Water level data for three wells on the western portion of the property were used to interpret a flow direction to the south, towards the Broad Beach site, for the western portion of the property. The geologic model was used to determine that the flow direction of groundwater in the eastern portion of the property is to the east, away from Broad Beach. Using the estimated volume of wastewater and a conservative approach of assuming that all of the wastewater from the proposed development will flow towards Broad Beach, which is inconsistent with our model, the impact of wastewater disposal on the Broad Beach water table was assessed. It was calculated that the rise of the water table in the Broad Beach area will be approximately 0.01m. This impact is minimal when the tidal influence on the water table (0.1m) is taken into account. The groundwater flow model developed in this study predicts that the true impact will be half of that determined in the worst-case scenario just presented. Previous Bedrock Surface Map 5 Geology The subject site is located in the southern foothills of the western Santa Monica Mountains. These mountains are the western extent of the Transverse Ranges that form a rugged east trending barrier of steep terrain between northern and southern California. The two most prominent geologic structures in the vicinity of the site are the inactive Escondido Thrust Fault and the active Malibu Coast Fault. Both of these structures are located north of the site, the farthest, the Malibu Coast Fault, is located approximately one mile away. The site is underlain by two types of non-marine terrace deposits, marine terrace deposits, older alluvium, and the Miocene age Trancas Formation Previous studies at the site assumed a continuous bedrock surface across the site that gently dips oceanward A detailed study of borings advanced at this location and adjoining locations shows that the bedrock is absent in the extreme eastern portion of the property at depths expected, but is once again exposed on the slopes east of Trancas Creek. Using boring log data, a new bedrock erosional surface map was developed. It is our hypothesis that an ancestral Trancas Creek cut a deep channel into the bedrock surface sometime prior to the deposition of the older alluvium on the site. New Bedrock Surface Map 1 A ground water flow map was developed using geologic data gathered during well drilling activities, ground water level data, and cross-sections developed during the study. It was determined that ground water flows in two different directions within the site. In both cases, ground water flow direction is governed by the semi-impermeable Trancas Formation. On the western portion of the property, ground water flows along the surface of the bedrock in a southern direction. On the eastern portion of the property, groundwater flows towards the south-east, down the steep slope of the bedrock surface. Conclusions Groundwater currently flows down gradient in the upper Trancas Formation and below the marine terrace sand deposits. The groundwater gradient was calculated for two flow directions based on the topography of the eroded bedrock surface. In the direction of the beach on the western side of the project site, it was calculated that the gradient would be 3.75 x 10-2 ft/ft due south. On the eastern side of the project site where the bedrock dips dramatically towards the channel is was calculated to be 0.108 ft/ft to the southeast. The groundwater velocity within the marine terrace sands and older alluvium is approximately 80.40 in/day (6.7 ft/day) towards the beach area. The flow towards the channel area and then towards the ocean has a velocity of 223.2 in/day (18.6 ft/day). The difference is associated with the variability in groundwater gradient, which, again, is a function of the eroded channel cut into the flat bedrock abrasion surface. The impact of wastewater on the existing water table and overall flow of water on and off the site were calculated. The calculations are based on City of Malibu projections of 950 gpd of wastewater for a five-bedroom house. Thirteen homes are proposed, for this study, on the Trancas Canyon property over the 14-acre area. Based on the primary marine terrace sand aquifer transmissivity under the present conditions on the subject site, if all the wastewater was discharged and moved through the 1,000 linear foot opening along PCH, the water table beneath the Broad Beach area would rise 0.40 inches with sustained flow. This is only one-tenth of the tidal variation of groundwater under the homes along Broad Beach Road as determined by Gorian and Associates, 1996. Transmissivity of the older alluvium under the eastern portion of the site was calculated to be approximately the same as that of the marine terrace sand, 380 gpd/ft2 based on slug test results. The developed flow model illustrates that flow should split fairly evenly between the east and west portions of the area and the actual impact of groundwater under the Broad Beach area should be approximately half that of the above calculated rise. Given the subsurface gradients and geological setting, wastewater will flow unimpeded through the marine terrace and modern beach sand commingling with the subsurface ocean water. The properties along Broad Beach Road currently have a separation of 5-10 feet between groundwater and the bottom of the wastewater disposal systems. As can be seen from the above discussion, only the small rise of 0.40 inches is expected to occur under these homes as a result of any proposed Trancas development. This rise will be masked by the tidal variations within the local groundwater. Three pneumatic slug tests were performed on wells TC-1, screened in older alluvium, and TC-2, screened in weathered Trancas Formation bedrock. Comparable hydraulic conductivities for the two wells were calculated from the tests. The site is located just west of Trancas Creek in the City of Malibu, Los Angeles County, California, on a bluff overlooking Broad Beach. Thirteen one-acre lots are planned for development on the property. Each lot will contain its own septic system used for waste water disposal. 4 Geologic cross-sections were developed for the site using the newly developed bedrock surface map (cross-section lines are shown in new bedrock map). Cross-section A-A’ traverses north-south across the site. In the northern portion of the property, the marine terrace sand deposit (Qtm), which will act as the conduit for waste water disposal is directly underlain by the south-dipping Trancas Formation (Tr). The Trancas Formation also slopes gently to the south in the western portion of the property. Cross-section B-B’ traverses east-west across the site. On the western portion of the property, the marine terrace sand deposit is once again underlain by the Trancas Formation, but on the eastern portion of the property, the terrace sand is directly underlain by older alluvial deposits (Qoal). The Trancas Formation has not been reached by any borings on this portion of the property. It is our interpretation that the Trancas Formation slopes steeply to the east in the eastern portion of the property. 2 Undisturbed soil samples were taken from the bottom of wells TC-2 (80’) and TC-3 (95’) and sent to a geotechnical laboratory to determine soil permeability of the bedrock. Laboratory results show that bedrock permeabiities are six orders of magnitude lower than those gathered from slug tests. This reveals that the bedrock is highly fractured and that slug test results represent fracture flow. Qtn Qts REFERENCES Campbell, R.H., et al., 1996, Geologic Map of the Point Dume Quadrangle, Los Angeles County, California. USGS. GeoConcepts, Inc., 2000, Limited Geologic and Soils Engineering Investigation, Trancas Market Renovation, 30745 Pacific Coast Highway, Malibu, California. Gorian and Associates, Inc., 1985, Additional Geotechnical Investigation, Future Waste Water Effluent Flow Patterns. Tracts 29273 and 32415 (Trancas I). Subsurface Designs Inc., 2001, Preliminary Geologic and Soils Engineering Investigation. Proposed Construction of Thirteen Single-Family Residences, Trancas Canyon, Malibu, California. Yerkes, R.F. and Campbell, R.H., 1979, Stratigraphic Nomenclature of the Central Santa Monica Mountains, Los Angeles County, California., Geological Survey Bulletin 1457-E Three groundwater monitoring wells were installed onsite to to determine flow parameters. An additional well located on the western portion of the site was also used for this study. 3 Ground water elevation data was also plotted against tide data for the Santa Monica Bay in order to determine if the system is tidally influenced. No correlation between tide data and water level data was discovered for wells TC-2 and TC-3. Well TC-1 (graph shown above), at a lower elevation than the other two, was determined to be tidally influenced. High water levels in the well were found be coincident with high tide levels 12 hours prior. Two non-marine terrace deposits overlay the marine terrace sand deposit onsite. Stream terrace deposits (Qts) were encountered in wells TC-2 and TC-3 on the western portion of the property. Non-marine coastal terrace deposits (Qtn) were encountered in Well TC-1 on the eastern portion of the property. These samples are described by Campbell (1996) as interbedded gravels, sand, silt, and clay. It was determined from lithologic logging of the wells, that the deposits differ in color. Qts deposits are darker and more red-brown than the tan-brown deposits of Qtn. Passive gamma ray logging of the wells also shows that the Qtn deposits are coarser grained than the Qts deposits. Both units are exposed along the bluff just south of Pacific Coast highway. Mapping of the bluff also revealed that the gravel fraction and percentage of the Qtn deposits is larger than that of the Qts deposits. Water level loggers were installed in each monitoring well in order to record temporal variations in ground water elevations. Precipitation data was also collected for the area and plotted with the water elevation data to determine the significance of recharge from precipitation. Special Thanks: Earth Consultants International, CSUF Graduate Students, Rene Perez and Sean Hunt.