Mohawk River

Recharge of a well by the Mohawk River, New York, has been demonstrated by Winslow et al. (1965) by studying hydrographs of the well and the river (Fig. 4.9) ... 

The close response of the well to fluctuations in the river level demonstrates that the well is dominantly recharged by the Mohawk River. Possible interference by pumping of adjacent wells may be ruled out on the basis of the monotonous abstraction, shown at the bottom of Fig 4.9. 

Changes in the water table of the Mohawk River and a number of adjacent observation wells is reported in Fig. 4.10, adapted from Winslow et al. (1965). The wells followed the river, with a time lag of 4-12 hours (insert in Fig. 4.10). Two possible explanations for this time lag may be envisaged (1) arrival of the hydraulic pulse, or (2) arrival of the recharge front (assuming piston flow section 2.1). To tell the two apart, the time lag observed for these wells by temperature measurements is helpful, as discussed in section 4.8 (see Fig. 4.21). The temperature time lag of, for example, well 58, has been observed to be about 3 months, whereas the water table time lag was only 12 hours. The latter defines the arrival of the hydraulic pulse, whereas the former defines the travel time of the recharge front. The distances given in the insert in Fig. 4.10, divided by the respective time lags, provided the... 

Fig. 4.9 Hydrograph of the Mohawk River and water table variations in an adjacent observation well (following Winslow et al., 1965). The perfect match (text) proves recharge from the river. Pumping from adjacent wells (bottom) was steady and could not cause the observed changes of the water table in the well.

Fig. 4.10 Water table fluctuations measured at wells near the Mohawk River (from Winslow et al., 1965). The wells followed a flood event with a time lag of several hours correlated with the distance from the river (insert table). The distance divided by the time lag provided the propagation velocity of the hydraulic pulse.

Tracing Groundwater by Temperature—A Few Case Studies The Mohawk River... 

Fig. 4.20 Monthly maps of equal groundwater temperature for a well field bordering the Mohawk River (after Winslow et al., 1965). The river temperature changed over an annual cycle from 77 to 32 °F. The aquifer followed these temperature changes, indicating recharge from the river.

A second way of presenting the data of the temperature survey at the Mohawk River is in the form of seasonally repeated depth profiles in a well (Fig. 4.21). The temperatures are seen to increase for half a year and then decrease, proving recharge, similar to the mode seen in the temperature maps (Fig. 4.20). The profiles show the vertical dimension of the recharge— temperature fluctuations are accentuated between 180 and 200 ft. Recharge is most efficient in this horizon, indicating highest conductance. 

Fig. 4.21 Seasonal temperature profiles in well 61, 100 m away from the Mohawk River (from Winslow et al., 1965). The temperatures are seen to decrease from October-March and to increase from June-September, similar to the trends seen in the temperature maps of the previous figure. The profiles reveal that the largest temperature variations occurred at a depth interval of 180-200 ft above sea level, indicating recharge occurred mainly through this part of the rock section, which in turn must have a higher conductivity.

A fourth way in which temperature data from the Mohawk River were processed is shown in Fig. 4.23. Temperature observations over a whole year are plotted for six wells and for the river. Wells 54 and 59 are seen to follow... 

Fig. 4.22 Location map of wells near the Mohawk River with annual groundwater temperatures (from Winslow et al., 1965). Using these values, contours of equal annual temperature variation were drawn. The decrease of these contour values indicates water moves from the river into the aquifer, especially through the zone marked A.

Fig. 4.23 Temperature records over a whole year in wells near the Mohawk River (following Winslow et al., 1965). Wells 54 and 58 follow the river temperature changes with a time lag. Well 21, most distant from the river, revealed a steady temperature over the year, indicating that river recharge is probably not contributing to this well. The rest of the wells showed intermediate degrees of temperature response to the river temperature variations in proportion to their hydraulic distances.

Figure 4.21 depicts temperature profiles in a well close to the Mohawk River, New York. In May a temperature of 43° F is observed at the measured depth interval of 205-165 ft above sea level. The July profile looked totally different it showed 57° F at the top, 64° F at the center, and 52° F at the bottom. The January profile looks different again 56° F at the top, 68° F at the center, and 52° F at the bottom. The interpretation of these temperature profiles is discussed in section 4.8. For our present topic, the planning of hydrochemical studies, it is important to keep in mind that water strata are not necessarily uniform—if they vary in their temperature profiles, they may vary in other parameters as well. 

Figure 10.10 provides data on repeated tritium measurements in a well and in the adjacent Mohawk River. What hydrological conclusions can be drawn The variations in tritium concentrations in the well followed variations in the river, as shown in Fig. 10.10. Hence, the river is recharging the well. The time lag in the well s response can be used to calculate recharge velocities. The data reveal piston flow of the recharge water, with little smoothing by dispersion (Fig. 10.10).

Winslow, J.D., Stewart, Jr., H.G., Johnston, R.H., and Crain, L.J. (1965) Groundwater resources of eastern Schenectudy County, New York, with emphasis on infiltration from the Mohawk River. State of New York Conservation Department Water Resources Commission Bull. 57, 148. 

Sloan RJ, Jock K. 1990. Chemical contaminants in fish from the St. Lawrence river drainage on lands of the Mohawk nation at Akwesasne, and near the General Motors Corporation central foundry division Massena, New York Plant. Technical report 90-1 (BEP). Division of Fish and Wildlife. 

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