Generally speaking, the cuttings and spent fluids from deep drilling contain some plant nutrients. It depends on the strata but commonly there is phosphate, maybe a little potash and some trace elements. Not much but rarely none at all. Why truck that somewhere else when it could be spread on the field next door? This is true whether it be deep drilling for geothermal heating, a search for water, trenching for a pipeline or searching for shale gas or oil.
By all means test for anything that might be dangerous if present and if it is, burn some diesel fuel in a truck on local roads, take it somewhere else and dispose of it by locking it up somewhere. On the evidence we have, generally speaking that will not be justified. But still do the testing and ask an independent body such as the British Society of Soil Science and/or the Institute of Geology to judge and to advise.
Bill Butterworth 10th April 2016