STABLE WATER ISOTOPE HYDROLOGY OF THE SANTA MONICA MOUNTAINS – A FIFTEEN YEAR SYNOPSIS

Numerous studies by the authors over the past 15 years have shed light on important hydrogeologic phenomena in residential and open space areas of the Santa Monica Mountains and southern Simi Hills, California. Studies extending from Thousand Oaks, to Calabasas, to Malibu provide many provisional interpretations that are of use in understanding the hydrology of urban and open space areas. Imported State Project water used almost exclusively in the area is easily distinguished from native water sourced from local precipitation. Important findings include the following: 1) There is an elevation effect where isotopically heavier precipitation falls out on the coastal, windward side of the mountain ranges (near Malibu) and isotopically lighter precipitation rains out at higher elevations and on the leeward side of mountains (near Calabasas and Thousand Oaks). 2) Imported water and recycled water have created recharge of shallow aquifers beneath several of the local cities, and streamflows in urban catchments show variation of stable water isotopes, with some urban streams containing almost all native water and other urban streams containing a mixture of some imported water and equal amounts or greater amounts of native water. 3) Recycled water contains a small amount of native water, probably due to leakage of groundwater into leaky sewer pipes that penetrate water tables locally. 4) Many storm drains flow continuously during dry weather conditions due to groundwater seepage into the drains. 5) Irrigation of urban lawns creates moderate to moderately limited isotope fractionation when tap water runs off into urban street gutters. 6) Water isotope signature varies significantly in individual rain events and between rain events, but there is a profound averaging effect of stable water isotopes during groundwater recharge. 7) Diurnal studies during dry weather demonstrate a similar averaging effect in urban streams at and several hundred meters below major storm drain inputs