WorldWide Drilling Resource

50 JULY 2016 WorldWide Drilling Resource ® Two Decades of Probing the Los Angeles Basin Adapted from Information Provided by the United States Geological Survey M ons of southern Ca forn ans re y on groundwater from the Los Ange es Bas n. For the ast two decades, the U.S. Geo og ca Survey (USGS) has been work ng w th oca water management agenc es to ga n a better understand ng of the groundwater f ow w th n the bas n, and create an mproved mode of the sedment ayers be ow ts surface. These efforts are prov d ng mportant deta s, not on y for susta nab e management of the reg on s groundwater supp y, but a so to he p ocate areas part cu ar y suscept b e to earthquake shak ng. The Los Ange es area s ocated on a sem ar d coasta p a n. H stor ca groundwater deve opment of the Centra and West Coast Bas ns caused a arge dec ne n groundwater eve s and ncreased seawater ntrus on. These effects ed to regu at on of the bas ns n the ear y 1960s, a ong w th the n t at on of a ser es of groundwater management act v t es, nc ud ng construct on and operat on of nject on barr ers, recharge of groundwater bas ns through spread ng grounds, restr ct on of groundwater w thdrawa dur ng certa n per ods, and use of mported and rec a med water n eu of pump ng n some nstances. The USGS, n cooperat on w th the Water Rep en shment D str ct of Southern Ca forn a (WRDSC), conducted mu t p e stud es. Most of these stud es have been focused on three perspect ves: (1) research dr ng and data co ect on, (2) geohydro og c and geochem ca ana yses, and (3) mode or database deve opment and app cat on. Dr ng operat ons and data co ect on has been a cruc a part of ongo ng research. As of Ju y 2012, the USGS had constructed 50 deep, mu t p e-we mon tor ng s tes throughout the Centra and West Coast Bas ns. Over the years, deta ed hydro og c and water qua ty data have been co ected from these s tes. The USGS has app ed geo og c and groundwater mode s to address a host of pract ca quest ons for the reg on: What are the ma n pathways for seawater ntrus on? What are the best strateg es for recharg ng the groundwater supp y, such as pump ng, nject ng, and spread ng water? Where are the fau ts ocated underground; and what earthquake hazards do they pose? Wh ch areas face greater r sks to fe and property dur ng earthquakes because they are more ke y to exper ence ntens f ed shak ng or so quefact on? An mportant aspect of dr ng n recent years has been the co ect on and nterpre- tat on of core samp es and advanced geo- phys ca ogs from the boreho es of each dr ng s te. Us ng the new nformat on, a ong w th the recent y acqu red se sm c ref ect on data, USGS sc ent sts and coworkers have been ab e to deve op a much more deta ed understand ng of the strat graphy and structure of the Centra and West Coast Bas ns. Geohydro og c/geochem ca ana yses and mode ng of recent years have been focused on the deve opment of a sequence- strat graphy-based, reg ona -sca e groundwater f ow mode . The mode a so nc udes a recharge sub-mode for watershed-w de, prec p tat on-runoff- nf trat on s mu at on. Future p ans for app y ng the reg ona groundwater f ow mode nc ude: (1) assessment of a - ternat ve water management strateg es, such as conjunct ve use and aqu fer storage and recovery, (2) dent f cat on of potent a f ow paths and d str but on of d fferent sources of water (seawater, mported water, rec a med water) after they enter the bas ns, (3) s mu at on of contam nant movement by coup ng the f ow mode w th a transport modu e, and (4) eva uat on of the mpacts of urban zat on on the groundwater system, such as an ncrease n pumpage, n ng of r vers and reduct on n perv ous areas, and ntroduct on of new sources of water nto the bas ns. The cooperat ve efforts of the USGS, the WRDSC, and the Los Ange es County Department of Pub c Works are prov d ng a better understand ng of the groundwater system and geo ogy of the Los Ange es Bas n. Th s know edge w be essent a to ensur ng adequate groundwater supp es for th s popu ous urban reg on and m t gat ng the threat posed by earthquakes. Top: Over a decade ago, the USGS collected sediment cores while monitoring wells were being drilled in the LosAngeles Basin. Bottom: Drill rig operators Bryan Sanchez (left) and Jack Hennagan (right) of the USGS Research Drilling Program and Hydrologic Technician Anthony Brown (center) from the California Water Science Center take a quick selfie break as they finish installing a 3023-foot hole which will house three groundwater monitoring wells in Lynwood, California. This is the deepest among 54 other well sites in the region. During earthquakes, ground shaking can cause water-saturated sediment to liquefy, resulting in ground failure. In the 1933 Long Beach earthquake, liquefaction of the underlying soil damaged this section of the Pacific Coast Highway south of Los Angeles. Photo courtesy of the City of Long Beach Public Library History Collection.