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Recent rains recharge underground water
MISSISSIPPI STATE -- Two years of drought lowered Mississippi's underground water supplies, but recent rains have stopped the loss and should be helping replenish formations.
Aquifers are underground water stores found in sand and gravel confined between layers of clay or solid rock. These geologic formations overlap each other at varying depths and extend for miles. Individuals and municipalities tap into these aquifers with wells to supply the population's water needs.
Jim Thomas, agricultural engineer with Mississippi State University's Extension Service, said recent years' drought has caused more water to be pulled from the aquifers and has not allowed them to be refilled.
"In most situations when an aquifer is not over-extended, you pump water out of it at a similar rate to what it recharges under normal conditions," Thomas said. "What we've missed is the recharge for the last two to three years."
Dr. Charles Wax, state climatologist and head of MSU Geosciences, said the drought is certainly over.
"It's hard to say when we're in a drought and equally hard to say when we're out of it, but we're definitely out of it now and we've had more rain than normal since December," Wax said.
Wax said a normal rain accumulation for Mississippi from November through March is 27.9 inches. In this most recent five months, the state has received 33.4 inches, or about 120 percent of normal. While December was drier than normal, each of the other months has exceeded averages.
"A lot of this excess will go to replenish the deficit the state has accumulated," Wax said. "There's a lot of water in the environment, and rivers and lakes are filling up as the rain is running off. There is even flooding in some areas. The soil moisture is about 100 percent recharged in most areas by now, and I imagine the shallow aquifers are already beginning to get some of the recharge."
Thomas said the speed at which aquifers recharge depends on the type of materials the water must move through. Water flows through sandy soil much faster than it moves through clay. And as it moves through the ground, it is naturally filtered.
"By the time it gets to the formation, the water is virtually clean," Thomas said.
Aquifers recharge very slowly. Sometimes the movement of water is measured in inches or feet per month or per year. The more saturated the soil is, the quicker these formations tend to refill.
"If we can keep the rivers and lakes full and the surface water systems full, the potential for recharge is much better," Thomas said. "What you get underground depends on the rain and amount of water on top of the ground."
The alluvial aquifer under the Mississippi Delta supplies most of the water for irrigation in this area of farmland. Thomas said the aquifer usually drops in the summer, but comes back by spring. For the last few years, the level has dropped and the lack of rainfall has not replenished it to normal levels.