Question

why there is a difference between the temperature of surface water and the ground surface

Answer 1

There is a difference between the temperature of surface water and the ground surface as  surface water includes the solid forms of water— snow and ice. The water below the surface of the Earth primarily is ground water, the ground water maintains a constant temperature, whereas the temperature of surface water alters according to the surroundings.

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Answer 2

The temperature of shallow ground water in the U.S. ranges from 44°F in the north central areas to approximately 80°F in Florida and southern Texas. In North America, geothermal heat pumps can operate efficiently at ground water temperatures as low as 39°F, or even lower, with equipment currently being constructed. Higher temperature ground water will, of course, put less demand on the system and make it more efficient

In the extreme South, where the cooling cycle predominates, ground water temperatures average about 72°F. It leaves the geothermal heat pump at about 85°F. Geothermal heat pumps have been used for many years in Florida.

In the North, where the heating cycle is crucial, ground water temperatures average about 52°F. There is less flexibility available in severe northern climates for lowering the ground water temperature before freezing occurs. Some geothermal heat pumps are designed to operate with a very small temperature drop in the water to avoid freezing damage. aThe temperature of water from wells is remarkably constant. In wells that are from 30 to 60 feet deep, water temperature is 2° to 3°F above the annual mean temperature of the locality. Water decreases in temperature about 1°F for each 64 feet of depth to the well.
In general, deep wells extend down through an impervious layer to reach an underlying supply. Shallow wells, in contrast, are sunk in easily penetrated strata to a point where they are below the water table. In terms of depth, deep wells are classified as those extending below 25 feet; those going less than this are considered shallow wells. Actually, deep wells vary from 100 to 3,000 feet. The vast majority are in the 100 to 1,000 foot range. Deep well water usually shows but slight change in composition over a long period of time. In one study of some wells in Florida over a 24 year period, hardness ranged from a high of 342 to a low of 304 parts per million. Alkalinity went from a high of 168 to a low of 148.