Factor analysis is useful for interpreting commonly collected ground‐water quality data and relating those data to specific hydrogeologic processes. One hundred nine ground‐water quality samples from wells completed in the upper Floridan Aquifer near Live Oak, Florida were analyzed for major dissolved constituents. R‐mode factor analysis was used to separate those chemical variables that reflect areally‐significant recharge processes from those related strictly to the dissolution of aquifer materials. Areas impacted by direct, rapid, artificial recharge through drainage wells and sinkholes, as well as by slow, natural recharge into the Floridan Aquifer, were delineated. Four factors which represent different chemical processes were identified and their relative areal impact determined. These processes are: (1) regional dissolution of the aquifer limestone, (2) dissolution and ion exchange in the discontinuous, semipermeable layer that overlies portions of the aquifer, (3) and (4) recharge from local, urban and agricultural runoff.
Hydrochemical facies interpretations are useful tools for determining the flow patterns, origins, and chemical histories of ground‐water masses. Factor analysis is advantageous for hydrochemical interpretations because it is independent of the number or type of variables used. Factor analysis also allows avoidance of problems of closed‐number systems inherent in more traditional techniques, such as trilinear diagrams. This paper applies factor analysis to the interpretation of mixing between sulfate and bicarbonate ground‐water masses. Whereas trilinear diagrams show one mixing trend (bicarbonate with sulfate waters), factor analysis allows interpretation of multiple mixing trends. These trends include the bicarbonate‐sulfate trend; a sodium‐, silica‐, fluoride‐, and temperature‐mixing system that is interpreted as resulting from recharge; and a chloride‐sodium system that appears to represent mixing with residual, connate water. The latter trends are identified as small‐scale, chemical variations that result in dispersion of the data points about the dominant mixing trend on the trilinear diagram. Thus, factor analysis provides greater precision in identifying hydrochemical facies and interpreting their origins.
A survey of waste‐migration patterns from septic‐tank/ tile‐field systems surrounding Houghton Lake, Michigan indicates that sampling plans designed to detect and quantify waste migration in ground water should be predicated on the concept that the waste plume may be complex and that the plume may not follow regional, ground‐water flow. The waste‐migration plumes at Houghton Lake range from simple, multichemical plumes that move with regional flow to complex plumes that bifurcate, that show different migration patterns for different chemicals, and that move up the regional gradient for short distances. The complexity of these patterns is attributed to a combination of the following system properties: loading rate and recharge at the waste source, local hydrology, chemical‐adsorption capacity of the soil, soil microbiology, regolith texture and fabric, and proximity to other waste sources. Based on the observed patterns, it is suggested that observation wells be placed so that an in‐depth, 3‐dimensional array of samples can be obtained. The wells should be of sufficient depth to insure that deep‐moving plumes can be detected and, if the actual, vertical‐migration pattern is of importance, the wells should allow collection of water samples at a number of depths. The waste‐migration pattern should be monitored throughout the year in anticipation of vertical movement of the plume axis during periods of surface recharge. If more than one chemical is of interest, then it is unsafe to assume that an index chemical, such as chlorides, demonstrates the migration of the other chemicals and analyses must be run for the other chemicals.
An investigation of waste‐migration patterns from a septic system indicates that complex patterns result from minor variations in regolith adsorptive capacity and texture, local hydrology, and possibly soil microbiology. The existence of multichemical, bifurcating plumes suggest that monitor wells arranged up and downgradient and capable of multilevel sampling are essential to adequately delineate contaminant migration in ground water. The data also indicate that sampling for a single constituent could yield misleading information about the nature and distribution of other ground‐water contaminants. The ability for chemical removal by the regolith is in direct response to minor variations in silt‐ and clay‐sized particle content and corresponds to Langmuir adsorption isotherms. Silt‐ and clay‐sized particles are dominantly organic in origin. Minor iron and aluminum hydroxyoxides and clays are present.
Substrate samples, when collected at regular intervals and analyzed for adsorbed constituents and textural variability, provide an integrated picture of the distribution of waste chemicals through time. Such samples also provide insight into the mechanics of plume configuration and flow characteristics within the regolith. The study shows that regolith adsorption data are essential to the determination of life expectancy of the regolith as a contaminant treatment system.
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