Various water quality parameters can influence the operation and treatment efficiency of adsorptive technologies (Amy et al. 2000, Chang et al, 2001, Clifford 1990). Termed "matrix effects", the combination of parameters such as pH, sulfate, phosphate, silica, TDS or other parameters in a raw water can greatly influence treatment performance. A greater understanding of matrix effects on the performance of adsorption media will help utilities select adsorptive systems that are more effective, robust, and cost-efficient. In order to evaluate "matrix effects" on sorption performance, a better understanding of the co-occurrence of arsenic and problematic species is required. Relating co-occurrence to geographic region would be particularly useful, since this will help develop a model water matrix that could evaluate the performance of adsorptive medias for a particular region of the country. The objectives of this ongoing work are to: characterize, from existing data, the probable co-occurrence of problematic water quality parameters as a function of geographic location, arsenic concentration and source water type; and, develop test water matrixes representative of "typical" and "difficult to treat" conditions by geographic region. Based upon the findings of Welch et al. (2000a), the study has simplified the occurrence of arsenic in the US into 3 geographic regions, specifically the Eastern, Central and Western regions. Within these regions, the occurrence of arsenic and co-occurring parameters will be determined from the USGS National Water Information System (NWIS) database. If possible, cumulative probability curves for co-occurring parameters will be developed by region to identify their range of concentrations. Statistical analysis will determine if any of these parameters correlate with the presence of arsenic within a region, or if significant differences in co-occurrences exist between regions. Using the results of analysis, sets of water matrixes will be created which can be used by utilities, media manufactures and researchers to compare the performance of alternative adsorptive treatment technologies under conditions representative of the geographic region defined above. Includes 9 references, tables, figures.