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No description available.

A marked change in ground water quality in the Safford Valley of Graham County, Arizona, averaging approximately +0.129 x 103 mhos electrical conductivity per year and +35 parts per million chloride per year, has been documented between 1940 and 1972 with data from ten long -term sample wells. A chloride change map constructed between these two years shows a general increase of 200 to 400 ppm chloride. The 1972 iso- chemical maps show areas of up to 1600 ppm chloride and 8.0 x 103 mhos electrical conductivity, which is extremely saline and considered threshold level for agricultural waters. The Safford Valley, a structural trough with approximately east -west orientation, averages 12 miles in width and 30 miles in length in the study area. Bounded by typical basin and range province mountains on the northeast and southwest, the valley contains a perennial stream flowing toward the west. A bi- aquifer system constitutes the ground water reservoir of the area with a deep, artesian aquifer of several thousand feet thickness overlaid by a water table aquifer averaging 400 feet in thickness and with the water table rarely over fifty feet from the surface on the eastern end of the valley, deepening to over 5000 feet at the western end. This bedrock -alluvium interface is the lower vertical constraint for the artesian system, thus the thickness of this aquifer increases downstream (to the west). The basin fill consists of a basal conglomerate overlaid by lacustrine evaporite beds, the aquifer cap beds, and recent alluvial material. The artesian aquifer is shown to be up to ten times as saline as the water table aquifer, and appears to increase in temperature and salinity in a downstream direction (corresponding to increasing thicknesses of lacustrine beds included in the extent of this aquifer). The decrement in the water quality of the surficial aquifer seems to be attributable to four major mechanisms. An increase in salinity may be expected from leakage of saline water from the artesian aquifer. Suck leakage would be stimulated by pumping- caused reduction of confining pressure, and by the puncture of the cap beds by deep wells. Water reaching the aquifer from natural recharge may contribute salts to the system. Such recharging water, if passed through soluble beds, could contribute to the salt content of the aquifer. Lateral movement of water through similar deposits may be a contribution, and the concentration and infiltration of agricultural water could also add to aquifer salinity. Ground water applied to the land surface is concentrated by evaporation and dissolves salts in the unsaturated zone as it re- enters the water table aquifer. Iso- salinity and salinity -change maps show the quality situation of the water table aquifer to be broken up into three major sections. From the eastern limit of the study area to Safford, the quality is relatively high and stable. From Safford to Pima there appears a uniform increase of low magnitude but continued decrement. Beyond Pima the area exhibits extremely irregular salinity conditions with marked increases and high salinity gradients. The salinity pattern corresponds to the extent of the underlying artesian aquifer but may be influenced to an unknown extent by the down- gradient transport of salts. The 1972 iso -chemical maps show chevrons of high quality water protruding into the aquifer at points corresponding to the locations of washes. Such wash bottoms are the principal zones of recharge in arid regions. Recharge from the Gila River is of extremely high quality relative to the salinity of the aquifer. There appear no configurations of iso -chemical lines which are attributable to internal movement through saline deposits. The hydraulic gradient of the water table aquifer is relatively constant and follows the gradient of the land surface. Concentration of irrigation water by evaporation and subsequent leaching while in conveyance to the water table seems to increase the salinity of this percolating water by approximately three -fold. The magnitude of this increase at any one point in space and time is a function of the volume of water applied to the land surface, the amount of evaporation, the initial chemical composition of the water, the chemical characteristics of the unsaturated zone through which it penetrates, and the transmission properties of the aquifer. The salinity increase seems significant but the extent of the contribution to the salinity of the aquifer is dependent on the amount of infiltration to the aquifer. This amount is currently undetermined, but is probably a sizable volume -- especially from pre- irrigation applications. A sociologic investigation based on responses from a detailed questionnaire - interview program of 41 farmers (25 percent of the farming population), indicated that there is an awareness of the high salinity of ground water being used for irrigation but relatively little concern about the rate of increase of that salinity. The farmers seem reluctant to leave the area and are willing to take somewhat greater economic losses than expected. Since the farmers of the area are principally Mormon, there may be a tie to this historically Mormon region which is stronger than usual. The economic analysis of the Safford Valley based on the modeling of a "Representative Farm" analog indicates that cotton will remain economical to produce on the basis of the projected salinity trends and ceteris paribus conditions, for a significant time beyond limits of prediction. The analysis indicates that the optimum salt-resistant crops for the area are being cultivated, and that of these, alfalfa, the least tolerant, will cease to be productive in large areas of the valley by 1990. The entire valley will not be able to economically produce alfalfa by 2040, but will remain in production since it is a necessary crop for cotton and the cotton profits should be sufficient to cover the alfalfa losses. Pumping is the only element in the operation of the social, physical and economic systems by which salinity change could be influenced significantly. The area east of Safford is the optimal pumping region while that west of Pima is the worst. The employment of surface water should be maximized, and salt- oriented field methods should be employed. Although agriculture does not seem in danger in predictable time, these practices would increase yield (or slow the decrease) and postpone the day when farming will no longer by profitable in the Safford Valley of Graham County, Arizona.

The concept of hydrochemical facies is used to study the distribution and, indirectly, to identify the origin of the chemical character of ground water in the basin-fill aquifer of the Tucson Basin in relationship to the hydrogeologic framework. Hydrochemical fades of ground water is defined operationally in terms of the lateral (horizontal) variation of chemical quality. The following chemical constituents are included in this study: total dissolved solids, calcium, magnesium, sodium, potassium, chloride, sulfate, bicarbonate, pH, nitrate, fluoride, strontium and zinc. A conceptual process-response model relates the regional distribution of dissolved constituents to the following hydrogeologic controls: (a) the chemical composition of the rock and soil in the drainage area before recharge to the ground-water basin and conditions at the recharge sites, (b) the lithology of the basin-fill aquifer, and (c) the direction of groundwater flow within the aquifer itself. Trend surface analysis suggests that the regional distribution patterns of total dissolved solids, calcium, sodium, sulfate and strontium show a tendency to parallel the direction of ground-water flow. The distribution pattern of chloride ions based on old analyses shows a trend opposite to the distribution pattern of chloride ions based on new analyses from the same area. Nitrate content of ground water and specific capacity of wells seem to be related. Q-factor analysis of data from the basin-fill aquifer demonstrates that the overall chemical character of the ground water does not change substantially as it moves through the basin. It is concluded that the chemical character of ground water in the basin-fill aquifer of the Tucson Basin was acquired mainly during contact with various rock types in the drainage basin before recharge. The lithology of the aquifer, presumably, only plays a secondary role in determining the overall chemical composition of the ground water. After recharge to the basin-fill aquifer the distribution of dissolved constituents is controlled primarily by the flow pattern. A chemical equilibrium model of calcite and water is used to approach the problem of determining whether precipitation or dissolution of calcite takes place in the aquifer. Measured calcium ion concentrations and pH values are compared to calcium ion concentrations and pH values computed for the equilibrium model. Assuming that the equilibrium model represents actual conditions in the aquifer, departures from the equilibrium model may be used to predict the chemical behaviour of calcite In the basin-fill aquifer.

Aquatic sediments, at a lake beach site in the Tonto National Forest, Arizona, were experimentally disrutped to quantitatively determine the impact of sediment resuspension on recreational water quality. Sediment resuspension was found to significantly degrade the quality of recreational waters. Mean sediment fecal coliform densities exceeded mean fecal coliform densities in overlying waters by 24.7 fold. When these sediments were experimentally disrupted, fecal coliform densities and turbidity levels in the water column increased by 1.4 to 2.8 and 1.1 to 5.4 fold, respectively. The impact of sediment resuspension on overlying waters was short-lived at the point of disruption. Increasing FC densities and turbidities generally peaked within 5 sec of resuspension and declined to preresuspension levels within 500 sec. The magnitude of the resuspension impact was most significantly influenced by water depth. The greatest risk to public health was found to be within the 0.3 m depth zone; that near-shoreline area most intensively used by young children for water play.

This investigation has documented the water quality impacts of stream channel recharge of sewage effluent northwest of Tucson and has evaluated the hydrogeochemical mechanisms potentially responsible for observed water quality changes. The evaluation was accomplished partly through construction of twelve monitor wells and implementation of a quarterly water quality monitoring program for surface water and groundwater. Constituents monitored included major inorganic chemical constituents, trace inorganics, trace metals, priority pollutants/trace organics and microbiological contaminants. ¹⁵N also proved useful in the study. The significance of a reducing zone immediately beneath the bed of the Santa Cruz River with respect to denitrification was documented. Findings of this investigation indicate that while sewage effluent is of poor quality with respect to drinking water standards, groundwater recharged by sewage effluent is of improved quality. Other findings include: * Nitrate in shallow groundwater near the Santa Cruz River in the Cortaro area appears to be primarily from stream channel recharge of sewage effluent; * Nitrate-N contents of effluent recharged groundwater averaged about 5 mg/1, well below the maximum contaminant level for drinking water of 10 mg/1, representing a 75 percent loss in total nitrogen during stream channel recharge of sewage effluent (assuming no mixing); * Both stream channel recharge of sewage effluent and agricultural deep percolation contribute to nitrate in shallow groundwater near the Santa Cruz River in the Marana area; * The reducing zone beneath the Santa Cruz River may be responsible for denitrification losses of up to 5 mg/1 of nitrate-N. This would explain approximately 30 percent of nitrogen losses, on average, between effluent and recharged groundwater; * The mechanisms of cation exchange and mixing with groundwater from other sources can explain the major changes in water quality between effluent and groundwater with respect to major inorganic chemical constituents; * Renovation of effluent with respect to coliform bacteria and enteric viruses content during recharge is not complete.