| The capture zone of a pumping well refers to a zone in aquifers where the pumped groundwater comes from within a specific time,which is a three-dimensional space.Delineating the capture zone solves the problem of where water comes from for a pumping well or where water flows to for an injecting well.It is a key factor needed to be considered in dividing groundwater source protection areas and remedying groundwater contamination.In 2018,the “Technical guideline for delineating source water protection areas” was revised and released in China,which required delineating catchment zones with the numerical modeleling method when dividing protection areas for a large groundwater source site.In fact,the geometric circle drawing method could be also invalid for small and medium water source sites and should be modified by considering the real characteristics of catchment zones.Piedmont alluvial-proluvial fans role as reservoirs of a large amount of high-quality groundwater and have been frequently selected as pore water source sites.Due to their special hydrogeological characteristics,the general geometric and empirical methods are generally insufficient to accurately delineate the capture zones.In order to scientifically determine the capture zone of pumping wells in alluvial-proluvial fans,relevant theoretical research are needed.According to the hydrogeological characteristics of piedmont alluvial-proluvial fans,this study developed ideal models at different levels,including: a segmented piedmont recharge model,a two-dimensional groundwater flow model with stream-water leakage and a three-dimensional groundwater flow model.The catchment zone of pumping wells was delineated through numerical simulation with MODFLOW+MODPATH.Shape factors were introduced to quantitatively handel the shape of a capture zone and the influences of control paramters on shape factors were analyzed.The works revealed the morphological characteristics and variaotion rules of catchment zones for pumping wells in alluvial-proluvial fans.This study discovered that the catchment zone of a groundwater source near a disconnect stream on one side could extend across the stream to another side,indicating that a pumping well could intercept groundwater from the opposite bank of a disconnect river.Therefore,it is inappropriate to treat large rivers as boundaries of groundwater flow systems in alluvial-proluvial fans.The crossing-type catchment zone beneath a disconnect stream is controlled by the pumping rate,the location of water source site,the width of the river and the leaking recharge rate.In this study,the theoretical research results were used to investigate the water source area in the Golmud alluvial-proluvial fan,Qinghai Province,China.A three-dimensional numerical model of the study area was established and capture zones of multiple water source sites were delineated.The results were compared with that from ideal models to reveal more features of the catchment zone in such an area.When groundwater is extracted from multiple water source sites in the same alluvial fan,the capture zones could form a superimposed nested state.By setting different groundwater exploitation scenarios,the evolution process of the multi-water source capture zones were analyzed in detail,as well as the way of how the the pumping rate at a water source site affects capture zones of other water source sites.The research could contribute new ideas on the utilization and protection of groundwater resources in the area of Golmud city. |
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