geothermal resource potential of the safford-san simon basin, arizona

may displace the last remnants of this lake to the location adjacent to the
~~W-oriented structure contours, thereby localizing the evaporite sediments.
As an analogy, sag ponds or small lakes sometimes occur on the downthrown
block adjacent to large Holocene faults.
Geohydrology
The blue clay apparently influences the geohydrology of the area.
Figure
28 is a piezometric surface map of the confined aquifer.
The shape of
the piezometric surface coincides with the shape of the structure contour
map at the base of the blue clay.
Steep contours on the piezometric surface
map indicate a damming effect on subsurface water flow to create rapidly
changing hydraulic pressure.
Low-permeability sediments or an impermeable
fault zone can cause ground water falls or steep hydraulic gradients.
Because
the blue clay thickens rapidly at these ground water falls, it is believed
that ground water flow is forced downward beneath the low-permeability
blue clay.
Estimated average temperature gradients of individual wells, using surface
discharge temperature, mean annual temperature and well depth, are plotted
in Figure 29.
Wells with temperature gradients less than 40 o C/km coincide with
ground water falls.
Downward-flowing water would transport heat, causing lower
temperature gradients.
Chemistry
Most thermal waters encountered in artesian wells are sodium bicarbonate
composition.
In the northern portion of the study area, sodium chloridesulfate
water with TDS up to 5,400 mg/l is encountered. Fluoride and boron
contents of these later waters are exceptionally high, with fluroide concentrations
over 25 mg/l.
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