Barry J. Hibbs

Ground Water Budget Analysis and Cross-Formational Leakage in an Arid Basin

Ground water budget analysis in arid basins is substantially aided by integrated use of numerical models and environmental isotopes. Spatial variability of recharge, storage of water of both modern and pluvial age, and complex three-dimensional flow processes in these basins provide challenges to the development of a good conceptual model. Ground water age dating and mixing analysis with isotopic tracers complement standard hydrogeologic data that are collected and processed as an initial step in the development and calibration of a numerical model. Environmental isotopes can confirm or refute a priori assumptions of ground water flow, such as the general assumption that natural recharge occurs primarily along mountains and mountain fronts. Isotopes also serve as powerful tools during postaudits of numerical models. Ground water models provide a means of developing ground water budgets for entire model domains or for smaller regions within the model domain. These ground water budgets can be used to evaluate the impacts of pumping and estimate the magnitude of capture in the form of induced recharge from streams, as well as quantify storage changes within the system. The coupled analyses of ground water budget analysis and isotope sampling and analysis provide a means to confirm, refute, or modify conceptual models of ground water flow.

Foreword: Ground Water in Arid Zones

An understanding of the motivation for this theme issue requires a brief review of early work on ground water in arid zones. Early work by the USGS reported assessments of arid ground water basins (e.g., Lippincott 1902a, 1902b; Slichter 1905; Mendenhall 1905, 1909; Meinzer and Kelton 1913; Meinzer 1917; Darton 1916). These studies of ‘‘parched lands’’ with ‘‘substantial subsurface storage’’ added to the inventory of water resources available for irrigation and municipal projects. C.V. Theis is best known for the development of the nonequilibrium formula for radial flow to a confined aquifer (Theis 1935), but he also carried out practical studies of ground water resources in New Mexico (e.g., Theis 1932, 1939). Owing to departures from the idealizations explicitly stated in the Theis equation, new analytical models were developed. One common departure was associated with leaky confined aquifers. Arid basins have semipervious interbeds of silt and clay that often ‘‘leak’’ when wells are pumped. Testing of basin fill units in southwestern aquifers probably provided part of the motivation for the development of leaky confined analytical models by C.E. Jacob and M.S. Hantush, who both held appointments at New Mexico Institute of Technology (Hantush and Jacob 1955). Studies of arid zones by UNESCO investigators during the post–World War II era were focused in North Africa, the Middle East, and Southeast Asia. A significant body of UNESCO’s work emphasized geochemical analysis of ground water. Under UNESCO’s charge, Schoeller (1959) observed that geochemical analysis not only is of practical importance in elucidating causes of salinization of ground water but also provides information on the storage and movement of ground water in arid zones. Schoeller’s report is a seminal piece of work. A solid record of ground water research was developed for arid zones in the Great Basin, Nevada, United States, from the late 1940s through the early 1970s (e.g., Maxey and Eakin 1949; Eakin 1966; Mifflin 1968; Winograd and Thordarson 1975). One major motivation of this work was linked to possible disposal of radioactive waste. Waste-related studies looked at hydrogeologic phenomena such as spatial variability of recharge, ground water residence times, and interbasin ground water flow. Limited horizontal and vertical well control in the Great Basin required extensive use of environmental isotopes and natural geochemical tracers for studying these processes. Isotopes and tracers are frequently used to study similar processes in arid zones today. Scientists investigating the Great Basin noted the existence of spring-formed travertine deposits at land surface, where contemporary ground water levels were tens of meters beneath land surface (Mifflin 1968). These travertine deposits dated from the pluvial periods of the late Pleistocene epoch and indicated that some aquifer systems achieved ‘‘flow capacity’’ several thousand years ago. Flow capacity is a condition whereby the water table is at land surface, having achieved its maximum rate of recharge based on a surplus of precipitation (Mifflin 1968). Recognition of deep water tables in undeveloped systems that were at flow capacity some 15,000 years ago led to concerns about waste disposal in arid zones. Water tables may rise or fall substantially in a few thousand years because of climate change. Buried wastes can then become inundated if the aquifer achieves flow capacity, which means that ‘‘safe’’ basins with thick unsaturated zones today may not be safe in the future. This represented early concerns about effects of climate change on arid zone ground water. Despite concerns about aquifer dynamics in the face of climate change, arid basins are targeted increasingly for storage or disposal of radioactive and hazardous wastes. To support waste disposal projects, a huge amount of vadose zone research has been completed in arid zones. Much of this work focused on identifying areas where precipitation and runoff water do not recharge the saturated zone. As thirsty desert cities grew and as undeveloped areas experienced new irrigation development, the need for accurate water budget analysis of regional aquifers became a priority. Analytical models were not adequate for developing regional water budgets in complex desert aquifers; analog models and later digital models provided Corresponding author: Department of Geological Sciences, California State University, 5151 State University Drive, Los Angeles, CA 90032; (323) 343-2414; fax: (323) 343-2435; bhibbs@calstatela.edu Copyright a 2008 The Author(s) Journal compilationa2008National GroundWaterAssociation. doi: 10.1111/j.1745-6584.2008.00444.x

Hydrologic Flux and Nitrate Exchange between Surface Water and Shallow Groundwater, San Diego Creek Watershed, Orange County, California

A study was conducted to assess water quality in the shallow aquifer of the San Diego Creek Watershed, Orange County, California. The objectives of the study were to determine concentrations of nitrate in surface water and shallow groundwater, and to study the hydraulic relationship between shallow groundwater and surface streams. San Diego Creek Watershed is located on the southern end of the Coastal Plain that covers a considerable po~on of Los Angeles and Orange Counties, California. The aquifers within the watershed are divided into shallow zones, which are not pumped, and deep zones, which are pumped. The shallow aquifer, which is in the upper 30 m of sediments, is composed of interbedded and discontinuous layers of silt, silty sand, sandy clay, clay, and sand. The shallow aquifer is hydraulically connected to a number of drainage ditches and channels that were constructed when the extensive marshland areas of the watershed were drained in the early 1900s. Some of these channels are lined with cement, and some are unlined and have sandy bottoms. Stream gaging along unlined channels indicates that flow accumulates as a result of groundwater seepage. Nitrate in shallow groundwater varies from about 40 mg/L to as much as 300 mg/L NO3. Sources of nitrate include contemporary and historical agriculture. Nitrate-laden groundwater seeps into and contaminates the surface streams in the watershed. Large commercial nurseries in the upper part of the watershed are another important source of nitrate. These nurseries release water with as much as 700mg/L NO3 into unlined surface channels. IAssistant Professor and 2Graduate Research Assistants, Department of Geological Sciences, California State University, Los Angeles; Los Angeles, CA 90032 (323) 343-2414; FAX (323) 343-2435; e-mail: bhibbs@calstatela.edu