2.5.2 EFFECTS OF WITHDRAWALS 



Large-scale and unregulated ground-water pump- 

 ing results in hydrologic problems such as declining 

 water levels, stream flow depletion, saltwater intrusion, 

 and land surface subsidence. 



When pumping is started in a well, the water table 

 is drawn down around the well to form a cone of de- 

 pression. The cone expands and the water table is pro- 

 gressively lowered until a balance is achieved between 

 the rate of flow of water to the well and the amount 

 pumped. If pumping rates continue to increase, the 

 size of the cone also increases. The creation of this 

 depression around a well or group of wells has led to 

 at least two documented effects in the Chenier Plain 

 and vicinity; saltwater intrusion and land subsidence. 



In the 210 m (689 ft) sands of the Chicot Aqui- 

 fer in Calcasieu Parish most of the ground water 

 moved gulfward prior to large-scale pumping opera- 

 tions. Because of the large freshwater head, saltwater 

 was flushed from the landward portions of the aqui- 

 fers. Because ground-water levels have declined in the 

 last few decades, the direction of the hydraulic gradient 

 has been reversed, the density balance has been dis- 

 turbed, and recharge with saltwater from the Gulf 

 has begun. The 210 m (689 ft) sands in central Cal- 

 casieu Parish now contain salty water as far north as 

 Lake Charles, and saltwater intrusion has caused many 

 industries to discontinue pumping operations from 

 this aquifer (Zack 1973). Decline in ground water in 

 the Gulf Coast Aquifer near Houston has also occurred 

 (fig. 2-9). 



The removal of water from the pore space of the 

 sands creates a void. Water from adjacent clay layers 

 moves into the interbedded sands. The dewatered 

 clays are highly compressible and become compacted. 

 In turn, the compaction is translated to the land sur- 

 face as subsidence. Ground-water and mineral extrac- 

 tion has led to a maximum of 2.5 m (8.2 ft) of subsi- 

 dence in northern Galveston Bay (Kreitler 1977). 



20 



30 



40 



SO 



80 



90 



.C100 



120 



. i I I I M I I I I I I I I I I I I I I I L M I I I I I M 



40 



44 



48 



52 



56 



60 



64 



68 



Figure 2-9. Depth to the water table in the Gulf Coast 

 Aquifer at Houston, Texas, 1939-72 

 (Kreitler 1977). 



23 



