Recharge rates to the shallow dolomite aquifer in northeastern Illinois are determined using a theory which describes the simultaneous transfer of heat and fluid in a porous medium. Temperature‐depth profiles in a nonpumping well are used to determine vertical groundwater velocities of the downward leakage of water recharging the aquifer which is overlain by semiconfining beds of clay‐rich glacial drift. Calculated and observed depths of penetration (19 m) of the annual surface temperature wave, and demonstration of the thermal stability of the well, validate application of the theory, which uses a dimensionless parameter β to relate curvature in temperature‐depth profiles to the magnitude of vertical leakage. Generally, the magnitude of β decreases as the depth interval L within the aquitard increases. The corresponding decrease in the vertical component of groundwater velocity accompanies a gradual coarsening with depth of the aquitard materials. Temperature differences over time at a given depth within a thermally stable well are found to be within the accuracy of the temperature measuring system. Analysis of variations in β Over time shows that the variability can be due to the limitations of the resolution (0.01°C) of the temperature measuring system. Recharge rates determined by analyses of temperature‐depth profiles range from 2.47×10−9 to 9.86×10−9 m/s and agree with estimates resulting from water‐budget method.

Clogging due to artificial recharge in laboratory-simulated unconsolidated aquifers displayed two types of patterns. The first type, resulting from recharge with turbid water containing an effective microbial inhibitor, showed clogging throughout the aquifers ranging in length from 48 to 123 cm. The rate of clogging at different depths was dependent on the size distribution of the particles in the water relative to the pore size distribution of the porous media. Clogging tended to be more severe at the infiuent portion of the sample; however, when the particles in the recharge water were smaller in size or when the pore sizes of the porous media were larger, the result was a'more uniform rate of clogging with respect to depth. The second type, resulting from recharge with nonturbid water and no effective microbial inhibitor, shows clogging only in the top few centimeters. If it is not understood that these two patterns result from different causes, such a pattern resulting from the combined factors could be ........... • to ordy one cause, namely, •1,5 bI'LD tl bt•l turbidity. Such a decision could lead to the conclusion that clogging due to turbidity will not penetrate deeply into the porous media, whereas if little microbial clogging is occurring, the result will be more clogging at greater depths. 1047

During the relatively mild winter of 1972–1973, 20,260 t (metric tons) of NaCl and 40 t of CaCl2 were spread on roads within the Salt Creek basin of the Chicago metropolitan area. Over 600 water samples were collected from 11 creek sites within the basin during the salting season. Chloride concentration in the water varied from 35 to 1530 mg/1 during the sampling period. Road salt chloride content of the water at any time is dependent on (1) temperature and colligative properties of NaCl, (2) duration of precipitation events, and (3) dilution capacity of the stream. Models are presented for three subsections of the basin based on the relationship between chloride concentration and discharge of the creek during nonsalting periods. A fourth subsection could not be modeled because of insufficient data. Monthly chloride budgets for all four subsections from November 1 through April 30 show that between 55 and 72% of the road salt chloride was removed by the creek from the various subsections. Changes in the percentage of chloride removed correlate well with the following indicators of the degree of urbanization: (1) percentage of area as streets, (2) highway density, (3) population density, and (4) road salt application per unit area. Sixty‐two percent of the chloride was removed by the creek from the basin during the first 6 months of the study period. Further sampling after April 30 but before salting commenced in December 1973 indicates that an additional 10% of road salt chloride was removed from the basin by the creek during the second 6‐month period. A portion of the remaining chloride was removed through the groundwater, and the rest was retained in the basin because of lag mechanisms.

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The knowledge of aquifer parameters is invaluable for quantitative description of geohydrologic systems and for optimal utilization of ground‐water resources. The classical methods of pumping‐test analysis are mostly graphical in nature and there is room for error in individual judgment in the geographical analysis. The computer method for aquifer evaluation comes under the inverse analysis techniques and is based on the classical principle of least squares. The sum of the weighted squares of differences between the observed drawdowns and the drawdowns calculated using the theoretical drawdown equations for the flow system under consideration is minimized, treating the aquifer parameters as decision variables. The method can be applied to any flow system for which analytical expressions for the potential distribution are known. The method was successfully applied to four pumping tests, two in nonleaky confined flow 1 systems, one in a leaky confined flow system, and one in an anisotropic nonleaky confined flow system. The percentage differences between the values of storativity and of transmissivity arrived at by the two methods are found to be as much as 24‐and 1.6 percents, respectively.

A comparison of the groundwater chemical data from the shallow dolomite aquifer of Northeastern Illinois Metropolitan Area (NIMA) from August 1972 through April 1973 with data collected 12–40 yr earlier indicates that concentrations of all ions have increased, the greatest increase occurring in the south‐eastern part of the area where the overburden is thinnest. The largest increases are 843, 142, and 91% in Cl−, SO42−, and Na+, respectively. However, the spatial distribution of the groundwater chemical types has remained the same with time. The groundwater was found to be apparently supersaturated with respect to calcite and dolomite; however, the two saturation indices were reduced by 20 and 30%, respectively, after considering the effect of ion pairings. The chemical character of the groundwater is controlled, among other factors, by the change of carbon dioxide content of the water along the groundwater flow path. A model for the solution of dolomite based on the standard equations for carbonate equilibrium is presented. The application of the model to the study area indicates that a variety of geologic conditions between the dolomite aquifer and the overlying glacial drift prevail in the area.

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