SEASONAL PUMPING VARIATION EFFECTS ON WELLHEAD PROTECTION AREA DELINEATION1

摘要

ABSTRACT:Delineation of a welihead protection area (WHPA) is the key element in welihead protection programs for drinking water supplies. WHPAs are often delineated under idealized conditions using simple steady‐state assumptions, which lead to an incorrect estimation of area and geometry. In this paper, we compare the results from a simple steady‐state procedure commonly employed in WHPA delineation with a more complex transient approach that allows consideration of seasonal variation in pumping rates. We also introduce a transient procedure to delineate time‐related capture zones using a numerical ground water flow and transport model. Welihead delineation is examined for two municipal wells in Tipton, Oklahoma, using a ten‐year time‐of‐travel criterion. In the steady‐state procedure, where we assumed constant pumping rates, we used GPTRAC, a semi‐analytical model, and MOC, a numerical model. The capture zone delineated by GPTRAC is comparable in shape with the capture zone delineated by MOC but not in size due to the differences in solution schemes. In the transient procedure, we used MOC and considered the seasonal variation in pumping rates. The capture zones delineated in this procedure were larger than the capture zones delineated by the steady‐state procedure using the same model. Further analysis showed that a higher drawdown was predicted in the transient procedure than in the steady‐state procedure, which is the reason for l