A study of the effects of excessive, long term irrigation on an unconfined groundwater system
Dixon, Deborah K
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Since 1937 the City of Lubbock has entered into a series of agreements with a private farmer, Mr. Frank Gray, to dispose of its treated effluent on a daily basis to Mr. Gray's farm. It is understood that the disposal will be in a manner acceptable to local and state health authorities. Mr. Gray, in turn uses the sewage effluent as irrigation water on his farm. In 1937, Mr. Gray received 1.0 to 1.5 million gallons per day (MGD) of treated sewage effluent on 200 acres. He considered this to be "good irrigation " , As the population of Lubbock increased, the quantity of the City's wastewater production also multiplied. Mr. Gray, however, was not able to acquire additional acreage at a rate sufficient to keep pace with the increase. In 1981, approximately 14.8 MG of daily average flow was applied to only 500 acres (three acre feet of water per acre per year). Quantities of effluent application have progressively become greater than the consumptive use of crops. Consequently, the Gray farm and adjacent acreage directly north of Yellowhouse Canyon and east of E. 34th Street have sustained a steady increase in groundwater level over the last 45 years. At this time, Lubbock Christian College Institute of Water Research (LCCIWR) and Texas Tech University are participating in a joint project funded by the Environmental Protection Agency to establish a demonstration land treatment system in a farm community approximately 20 miles south of the City of Lubbock. Upon completion of construction of this project, a maximum of 7.4 MGD of Lubbock's treated effluent, normally received by Mr. Gray, will be diverted to the alternate farming area. This will result in an immediate reduction of almost 50 per cent of the flow that Mr. Gray is currently receiving. The purpose of the research described in the following pages is to determine the effects, if any, by computer simulation of this reduction of irrigation flow on the water table elevation in the Gray farm area and to ascertain whether any undesirable consequences will result from this change. Through computer simulation of the groundwater formation underlying Gray's farm, water table elevations were projected for a thirty-three year period (1978-2010). Results of this simulation indicate a steady decline in the groundwater surface at a rate of approximately 0.01 to 0.64 ft./year (0.0 to 0.25 meters/year). The consequence of this water table elevation decline over the next thirty years will probably be advantageous to Mr. Gray. Indirectly, however, Buffalo Springs Lake may suffer a reduction of inflow from Yellowhouse Canyon resulting from the subsidence of springs and seeps from the canyon wall bordering the Gray farm southern boundary .