Water Resources of St. Charles Parish, Louisiana

How much water St. Charles Parish, Louisiana, has, how it is used and how good it is — from a U.S. Geological Survey fact sheet prepared with the Louisiana Department of Transportation and Development, drawing on earlier reports and USGS records.

How the water is used

In 2010 the parish withdrew about 2,476 million gallons a day (Mgal/d): about 2,471 Mgal/d of surface water and 4.24 Mgal/d of groundwater. Power generation took about 79 percent.

By source, the Mississippi River supplied 2,471.23 Mgal/d and smaller streams 0.05. Groundwater came from the Gonzales-New Orleans aquifer (3.74 Mgal/d), the Norco (0.48), the Gramercy (0.02) and point-bar deposits (less than 0.01).

Counted every five years, total withdrawals generally rose from 1960 to 2005, then fell by about 20 percent by 2010, mostly because industry drew less surface water.

Groundwater

The main freshwater aquifers — chloride of 250 milligrams per litre (mg/L) or less — are, from shallowest to deepest, the Gramercy, the Norco and the Gonzales-New Orleans. Mississippi River point-bar deposits hold a little fresh water, generally high in iron and very hard, and little is pumped. Deeper aquifers hold only saltwater.

The base of fresh groundwater ranges from about 350 feet below the National Geodetic Vertical Datum of 1929 (NGVD 29) near Des Allemands to more than 700 feet about 2 miles south of the Lake Pontchartrain shore; the southern half of the parish has little or no fresh groundwater. The aquifers are recharged by rain soaking in north of the parish, where they reach the surface, by leakage between aquifers, and from the Mississippi through point-bar deposits. They drain by leakage to neighbouring aquifers, into rivers and lakes, and to wells.

Gramercy aquifer

Fresh in western St. Charles Parish and in a small area near Norco and Hahnville, this fine-to-coarse sand is the least continuous of the three aquifers, thin or missing along Lake Pontchartrain. North of a line from Lac des Allemands to Lake Cataouatche its top is generally 100 to 250 feet below NGVD 29, and it is from less than 100 to 150 feet or more thick. Near Hahnville it connects with a Mississippi point bar, and its water levels rise in spring and fall in autumn with the river.

In 2014 records listed 27 active wells — 12 domestic, 9 irrigation, 3 industrial, 3 public supply — 60 to 357 feet deep, yielding 25 to 500 gallons per minute. Withdrawals in 2010 were about 0.02 Mgal/d. Its water varies with leakage from other aquifers: where unaffected, a very hard mixed calcium-magnesium-bicarbonate and sodium-chloride water, turning sodium-chloride downdip.

Norco aquifer

Fresh in the northwest of the parish, its top generally 250 to 400 feet below NGVD 29 and its thickness from about 25 to 275 feet, mostly well-sorted medium to coarse sand near the river. As pumping fell, water levels near Norco rose from the 1950s to the mid-1990s — about 60 feet from 1958 to 1995 in well SC-14.

In 2014 records listed 26 active wells — 14 industrial, 8 irrigation, 4 domestic — about 320 to 460 feet deep, yielding 175 to 2,000 gallons per minute. Withdrawals in 2010 were about 0.48 Mgal/d, nearly all industrial.

Its water ranges from sodium-bicarbonate to sodium-chloride. Where fresh, hardness is 40 to 60 mg/L as calcium carbonate, rising above 500 mg/L with salinity; iron is generally below 500 micrograms per litre; dissolved solids run from about 750 to 1,000 mg/L when fresh to more than 2,500 when salty. Colour ranges from 5 to 180 platinum-cobalt units — 50 to 100 at Norco — above the EPA's secondary limit of 15.

Gonzales-New Orleans aquifer

Salty in the southern half of the parish, and fresh over saltwater in the northern half. Its top lies from about 450 feet below NGVD 29 near Lake Pontchartrain to more than 800 feet near Lake Cataouatche; it is about 175 to 325 feet of fine to very fine sand, dipping south 25 to 50 feet per mile. Pumping lowered water levels near Norco in the early 1950s; in well SC-9 they were stable from 1972 to 1982 apart from brief swings, and stood about 20 feet below NGVD 29 when monitoring ended in 1995.

In 2014 there were 3 active wells — industrial, irrigation and domestic — 670 to 754 feet deep; one reported 1,293 gallons per minute. Withdrawals in 2010 were about 3.74 Mgal/d, almost all industrial. Fresh water here is soft, a mixed sodium-bicarbonate-chloride type with hardness of about 10 to 40 mg/L, becoming harder sodium-chloride water (above 300 mg/L) as it grows saltier with depth and distance; colour is generally above the secondary limit.

Surface water

The Mississippi supplies nearly all surface water: in 2010 about 2,470 Mgal/d, including about 1,960 Mgal/d for power generation and 503 Mgal/d for industry, plus public supply and livestock. Most power-plant and industrial water was used for cooling and returned to the river. No withdrawals were reported from Lake Pontchartrain, Lake Cataouatche, Lake Salvador, Lac des Allemands or Bayou des Allemands.

The Mississippi River

The river drains more than 40 percent of the continental United States, so rain, erosion, dams, diversions and runoff from farms, towns and industry upstream shape its flow and quality, which change with the seasons. Pesticides and nutrients usually peak in June and July, the "spring flush"; suspended sediment is highest in late winter and early spring. At Baton Rouge (site 07374000) record flows were 1,436,000 cubic feet per second in May 2011 and 132,800 in October 2012. At Red River Landing, about 175 river miles upstream of Norco, the 1928–76 average was about 460,000 cubic feet per second (about 298,000 Mgal/d) — representative of the parish, since no major tributary or distributary lies between and the river is mostly leveed.

Samples at Luling (site 07374400) from 1957 to 1999 show hard water that usually stays within the EPA's secondary limits for pH, chloride, sulfate and iron, with dissolved oxygen generally above 5 mg/L, the minimum for a varied community of warm-water fish.

Two structures divert river water. The Bonnet Carre Spillway, on the north bank between Laplace and Norco, is a 7,000-foot-wide flood-control structure that can send up to 250,000 cubic feet per second through a 5.7-mile, 8,000-acre floodway into Lake Pontchartrain. The Davis Pond Freshwater Diversion Structure, on the south bank about 2 miles east of Luling, uses culverts and pumps to carry up to 10,650 cubic feet per second of fresh water, nutrients and sediment into nearby wetlands, to slow land loss, help vegetation and improve fish and wildlife habitat.

Lakes

Lake Pontchartrain covers 621 square miles and averages about 11 feet deep; connected to the Gulf of Mexico, its salinity varies greatly, and its northwestern and western parts may be fresh for part of the year. Lac des Allemands covers about 23 square miles and averages about 5 feet deep; its quality varies by place and season, and chloride is moderately high, occasionally above the 250 mg/L secondary limit.

Sources

• White, V.E., and Prakken, L.B., 2015, Water resources of St. Charles Parish, Louisiana: U.S. Geological Survey Fact Sheet 2014–3118, prepared in cooperation with the Louisiana Department of Transportation and Development. https://pubs.usgs.gov/publication/fs20143118
• The tables and figures come from the fact sheet's PDF, which the web version lacks.
• Rewritten in hubnx's own words.