Water Resources of St. James Parish, Louisiana

How much water St. James 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 192 million gallons a day (Mgal/d): about 190 Mgal/d of surface water and 2.85 Mgal/d of groundwater. Industry took 92 percent of the total; the rest went to public supply, rural homes, livestock, general irrigation and aquaculture.

By source, the Mississippi River supplied 177.65 Mgal/d and smaller streams 11.88. Of the groundwater, 2.84 Mgal/d came from the Gramercy aquifer, 0.01 from the Mississippi River alluvial aquifer and less than 0.01 from the Norco aquifer.

Industrial surface-water withdrawals have dominated since 1970; counted every five years since 1960, the total peaked in 1980.

The aquifers

The aquifers holding fresh water are, from shallowest to deepest, the Mississippi River alluvial, the Gramercy and the Norco. Deeper aquifers generally hold only saltwater — chloride above 250 milligrams per liter (mg/L) — although the deeper Gonzales-New Orleans aquifer has small areas of fresh water over saltwater in the northeastern and north-central parish. Point-bar deposits of the river and local unnamed shallow sands above the Gramercy can also give some fresh water, but they are usually small, and their water is very hard and high in iron (above 300 micrograms per liter, µg/L).

Every freshwater aquifer except the Gonzales-New Orleans belongs to one connected system. Point-bar deposits, the alluvial aquifer and shallow sands often link the Gramercy and Norco directly to the Mississippi, and west of Convent the alluvial, Gramercy and Norco aquifers merge. The Gramercy and Norco are told apart mainly by their place in the sequence. The Gonzales-New Orleans is easier to recognise: its texture is uniform, and a clay layer up to 150 feet thick in the middle of the parish separates it from the Norco.

Because of these links, groundwater levels rise and fall with the river. Groundwater generally flows toward the river and drains into it, but when the river is high enough, or pumping has drawn levels down, river water recharges the aquifers. Most fresh recharge, though, comes from rain soaking into the ground.

Mississippi River alluvial aquifer

It holds fresh water in roughly the southwestern half of the parish. The upper part is fine to medium sand, the lower part coarse sand and gravel, and the base lies as deep as about 275 feet below land surface. It can yield several thousand gallons a minute of water that is generally hard and high in iron.

In 2014 state well-registration records listed 16 active wells in it — 2 domestic, 3 industrial, 8 irrigation and 3 public supply — about 100 to 212 feet deep, with reported yields of 10 to 2,300 gallons per minute.

Gramercy aquifer

It holds fresh water in an irregular band from near the parish's northwestern corner to its south-central part, and in a much smaller area near the town of Gramercy; it is thin or missing in the north-central and northeast. Fine sand at the top grades to coarse sand and fine gravel lower down. The top lies about 175–200 feet below the National Geodetic Vertical Datum of 1929 (NGVD 29), and the aquifer is from less than 75 to more than 225 feet thick, generally thickening from north to south.

Its water levels peak in spring and bottom out in fall, with the river's seasonal high and low stages; the swing shrinks with distance from the river.

In 2014 records listed 69 active wells — 25 domestic, 21 industrial, 19 irrigation and 4 public supply — about 200 to 348 feet deep, with reported yields of 10 to 3,260 gallons per minute.

Its water varies with inflow from neighbouring sands and aquifers. It is generally hard to very hard, a calcium-magnesium bicarbonate type, and hardest where the aquifer connects with point-bar deposits or shallow sands — except near Belmont, where a small area has soft water. It usually exceeds the U.S. Environmental Protection Agency's Secondary Maximum Contaminant Levels (SMCLs) for colour, iron, manganese and dissolved solids.

Samples from 70 Gramercy wells, 1931–2005:

Hardness, in mg/L of calcium carbonate: 0–60 soft, 61–120 moderately hard, 121–180 hard, above 180 very hard.

Norco aquifer

The Norco holds saltwater in most of the parish. Fresh water is found only in a narrow lobe from the northwestern border as far south as St. James. The aquifer is about 75 to 200 feet thick, and its top lies from about 250 feet below NGVD 29 in the northeast to 450 feet in the south. Like the Gramercy, it grades from fine sand to coarser sand and fine gravel. Its water levels also follow the river's stage, through an indirect connection.

In 2014 records listed 16 active wells — 7 domestic, 6 industrial and 3 irrigation — about 340 to 528 feet deep, with reported yields of 205 to 3,000 gallons per minute. The few samples taken from 12 freshwater wells in 1938–89 ranged in hardness from 40 to 420 mg/L as calcium carbonate, in pH from 6.5 to 8.5 and in chloride from 7 to 230 mg/L.

Surface water

In 2010 the parish withdrew about 190 Mgal/d of surface water: about 175 for industry, 11.8 for aquaculture, 3.12 for public supply and 0.06 for general irrigation. All industrial and public-supply water came from the Mississippi. Most of the industrial water was used for cooling and then returned to its source.

Natural high ground and levees along the river keep the Mississippi largely apart from the rest of the parish's surface water. Rain runs off the higher ground by the river toward lower land through a web of canals and streams. North of the river the parish lies in the Lake Maurepas Basin (Hydrologic Unit Code 08070204), which drains north and east through Bayou des Acadiens and Blind River toward Lake Maurepas. South of it, in the East Central Louisiana Coastal Basin (HUC 08090301), water drains south and east through Bayous Verret, Citamon and Chevreuil toward Lac des Allemands.

Smaller streams

Little data exist for them; discharge statistics could not be computed.

The Louisiana Department of Environmental Quality treats 5 mg/L of dissolved oxygen as the minimum for a varied population of freshwater warm-water life, sport fish included.

The Mississippi River

The river divides the parish into north and south and is its main source of fresh surface water. It drains more than 40 percent of the continental United States, so rain, erosion, dams and diversions upstream shape its flow and quality, which change with the seasons.

Its flow varies enormously. At Baton Rouge, about 54 river miles upstream of the parish line (site 07374000), the record flows were 1,436,000 cubic feet per second in May 2011 and 132,800 in October 2012. At Red River Landing (site 07373291), about 125 river miles upstream of Union, the average in 1928–76 was about 460,000 cubic feet per second — about 298,000 Mgal/d. That stands for the parish's flow: no major tributary or distributary lies between, and the river is mostly leveed.

Upstream farms, towns and industry also shape its quality. Pesticides and nutrients usually peak in June and July, the "spring flush" of runoff from upstream fields. Suspended sediment is usually highest in late winter and early spring and lowest in late summer and fall. Samples at Union (site 07374220) in 1973–93 show hard water that usually stays within the SMCLs for pH, chloride, sulfate and iron, with dissolved oxygen generally above 5 mg/L.

Sources

• White, V.E., and Prakken, L., 2015, Water resources of St. James Parish, Louisiana: U.S. Geological Survey Fact Sheet 2015–3038, prepared in cooperation with the Louisiana Department of Transportation and Development. https://pubs.usgs.gov/publication/fs20153038
• The tables, figures and captions come from the fact sheet's PDF, which the web version lacks. The aquifer areas on the map are modified from Tomaszewski (2003) and the cross-section from Griffith (2003).
• The fact sheet's manganese range for Bayou Chevreuil is given in milligrams per liter, unlike its other manganese figures, so it is left out here.
• Rewritten in hubnx's own words.