Sheryl Luzzadder-Beach George Mason University Accurately mapping a region’s ground water quality depends upon the spatial sampling strategies employed, including where and how often field data are collected. This study compares the relative values of three field sampling strategies for mapping a known migrating plume of volcanic ground water in Sierra Valley, California. The first strategy sampled wells once each year during 1957, 1972, and 1980 (n=63, 45, and 57, respectively) and portrayed spatial-temporal changes in ground water quality more clearly on maps than did two alternative sampling strategies. One of these alternatives, Strategy 2, sampled one well per township per year during 1957, 1972, and 1980 (n=11) and did not detect the migrating plume, despite being a recommended strategy. The other alternative, Strategy 3, frequently sampled in time a small, fixed group of indicator wells (n=13) every four years for the same period, again producing maps with little correlation to the original pattern detected by Strategy 1. Key Words: ground water, water quality monitoring, spatial- temporal sampling, water quality maps, water management. Introduction G round water is the most extensive fresh water reservoir on earth after glacial ice. It is certainly our largest accessible source of water for drinking and other uses. The quality of this important global resource varies greatly across space and time, and it is well within the geogra- pher’s realm to be concerned with the spatial dimensions of ground water quality. Little has appeared in mainstream geographical literature concerning this important resource (Beach 1990, 1992), despite the research by water re- sources geographers published in the environ- mental and hydrologic literature (Rajagopal 1986, 1987; Loaiciga 1989; Tobin and Rajagopal 1990; and Luzzadder-Beach 1995, among oth- ers). Many areas of the world, including parts of the western United States, regularly endure sur- face water deficit and are therefore increasing their dependence upon ground water. This situ- ation underscores the importance of refining geographic methods to determine and monitor the quality of ground water resources. A key challenge to assessing ground water quality accurately is determining the most ap- propriate field sampling design for mapping ground water quality. Previous research has studied two important considerations of best sampling designs: the most efficient number of sampling locations in a ground water basin and geographic pattern of these locations (Ward 1981; Nelson and Ward 1981; Pfannkuch 1982; Rouhani 1985, 1986; Rajagopal 1986, 1987; Beach 1987, 1990; Hsueh and Rajagopal 1988; Loaiciga 1989; Natarajan and Rajagopal 1994; Luzzadder-Beach 1995). A third consideration that has received less attention is the temporal dimension in spatial sampling of ground water quality, including both the time intervals be- tween sampling events, and measuring patterns of water quality change over time. This paper will focus on which geographic sampling strategies enhance our ability to de- tect and map changes in ground water quality over time. It analyzes a known, migrating plume of volcanic ground water in Sierra Val- ley, California, to test the effectiveness of sev- eral spatial sampling strategies for detecting a time-series of ground water quality changes in a real world setting. Unwin (1981) reminds us of the special difficulties of spatial sampling in geography: Whatever the underlying true correlation, the im- position of data collection zones leads to very different values for the correlation coefficient. It is *This work was funded, in part, by a summer research fellowship from the University of Minnesota’s Geography Department. The author wishes to thank the California Department of Water Resources for field research opportunities and data in connection with the Sierra Valley Ground Water Study. The author is indebted to the anonymous reviewers for their helpful suggestions. Special thanks go to T. Beach, D. Brown, P. Gers- mehl, H. Pfannkuch, R. Skaggs, and J. Wood for comments on earlier drafts of this paper, and to P. Cotterill for assistance with final graphics. Views expressed are the author’s and do not necessarily represent those of the aforementioned institutions or individuals. Professional Geographer, 49(2) 1997, pages 179–192 © Copyright 1997 by Association of American Geographers. Initial submission, December 1995; revised submissions, June 1996, November 1996; final acceptance, November 1996. Published by Blackwell Publishers, 350 Main Street, Malden, MA 02148, and 108 Cowley Road, Oxford, OX4 1JF, UK. Spatial Sampling Strategies for Mapping a Volcanic Ground Water Plume in Sierra Valley, California*