Recent increases in the use of hydraulic fracturing (HF) to aid extraction of oil and gas from black shales have raised concerns regarding potential environmental effects associated with predictions of upward migration of HF fluid and brine. Some recent studies have suggested that such upward migration can be large and that timescales for migration can be as short as a few years. In this article, we discuss the physical constraints on upward fluid migration from black shales (e.g., the Marcellus, Bakken, and Eagle Ford) to shallow aquifers, taking into account the potential changes to the subsurface brought about by HF. Our review of the literature indicates that HF affects a very limited portion of the entire thickness of the overlying bedrock and therefore, is unable to create direct hydraulic communication between black shales and shallow aquifers via induced fractures. As a result, upward migration of HF fluid and brine is controlled by preexisting hydraulic gradients and bedrock permeability. We show that in cases where there is an upward gradient, permeability is low, upward flow rates are low, and mean travel times are long (often >106 years). Consequently, the recently proposed rapid upward migration of brine and HF fluid, predicted to occur as a result of increased HF activity, does not appear to be physically plausible. Unrealistically high estimates of upward flow are the result of invalid assumptions about HF and the hydrogeology of sedimentary basins.
1)Rivers in hilly region are generally swift fast moving, steep and varying slopes, shallow depth and narrow as compared to rivers in plain regions. They are typically characterized by the presence of water falls of small to medium height, the algal growth attached to river bed stones, and small deeper pools formed in the course of river, which affect the rate kinetics of river. Thus, in these rivers significant re-aeration is caused by frequent falls and turbulence created by bed-stones; algal growth on bedstones causes large diurnal DO variations; pools formed in the river course act as oxidation ponds which remove significant BOD and formation of thick biological slime layer on the stones and rocks at the bed in the more polluted stretches. The widely used 1-diemnsional river water quality models such as QUAL2e and STREAM do not explicitly simulate these processes with the result that these models become less useful for modelling such rivers. Application of a simple 1-D water quality model was examined for a shallow and swift hilly river (Hathli Stream, Hamirpur, HP) wherein effect of benthic and attached algal photosynthesis, reaeration due to water falls, and deoxygenation due to small pools formed in river course were considered for a realistic simulation. It was observed that these factors significantly affect model calibration parameters and thus need to be incorporated in models for such rivers.
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