STABLE ISOTOPE EVIDENCE FOR GROUNDWATER SOURCES AND CIRCULATION IN THE PORTUGUESE BEND LANDSLIDE COMPLEX, RANCHO PALOS VERDES, CALIFORNIA

Oxygen and hydrogen isotope data acquired from 31 water samples collected at 14 monitoring wells in the inactive part of the Portuguese Bend Landslide and the active Abalone Cove Landslide reveal groundwater circulation patterns within this complex landslide. Three stages of sample collection were carried out: (1) the early part of the Southern California rainy season, Nov.-Dec. 2009, (2) the mid-rainy season, Feb. 2010, and (3) the middle of the dry season, Jul. 2010. This sampling strategy was employed to assess the influence of winter precipitation on the recharge process. This study is part of an ongoing hydrogeologic monitoring of this complex of landslides, and is the first of its kind that employs stable isotopes to constrain fluid sources during activity associated with slow rates of mass wasting, and will be considered in the abatement process. The observed range of groundwater δ¹⁸O values is between –6.9 to –5.6‰, values within the range of local San Gabriel River water (δ¹⁸O = –6.3‰, δD = –45‰, Holk and Kiseleva, 2007). This local source is confirmed by δD values between –49 to –32‰. Oxygen isotope values decreased only 0.3 to 0.7‰ between the beginning of the wet season and its end, consistent with slow migration within the aquifer. Shifts in δD values during this time were more variable with some samples becoming enriched in D/H (up to +6.7‰) while others display depletions in D/H (as much as –5.6‰) relative to the Nov.-Dec. 2009 δD value. Local recharge involving imported California State Water Project water (δ¹⁸O = –8.3‰, δD = –60‰) or Colorado River water (δ¹⁸O = –12‰, δD = –100) is suggested by a very low δ¹⁸O (–10.0‰) and δD (–82‰) value recorded from one well located in a highly irrigated area. All isotopic data plot right on the Meteoric Water Line, suggesting rapid infiltration and minimal evaporation. Numerous changes in the water table were recorded during this time, with some wells going dry between the end of the rainy season in late April and our most recent round of sampling in July 2010. Our model for the groundwater system active in the Portuguese Bend Landslide involves variable amounts of recharge across the entire area, downward percolation of water to multiple impermeable bentonite slip-surface layers in the landslide, followed by transport to, likely submarine, discharge sites at the landslide toe.