Effluent volume and pollutant loads at an inland, low-salinity, shrimp farm in Alabama

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Abstract

An inland, low-salinity shrimp farm in Alabama released 50–60% of stored water during harvests in 2008 and 2010. Maximum concentrations of potential pollutants occurred in the final 5% of water discharged from ponds during harvest. Average, weighted concentrations of variables were lower than maximum values, but greater than concentrations in the receiving stream upstream of the farm outfall. Nevertheless, chloride was the only variable for which the concentration found downstream of the farm outfall exceeded the permissible concentration limit of the Alabama stream classification system. Pollutant loads per tonne of shrimp for the 2 years averaged were 444.9 kg total suspended solids, 51.3 kg 5-day biochemical oxygen demand, 15.2 kg total nitrogen, 1.33 kg total phosphorus, and 4402 kg chloride. Concentrations and loads of most potential pollutants could be lessened by passing farm discharge through a settling basin before final discharge, releasing less water during harvest, and improving the feed conversion ratio.

Highlights

► An inland shrimp farm discharged about 50% of pond water during harvest. ► Water discharged had elevated concentration of several potential pollutants, especially dissolved salt. ► Nitrogen and phosphorus discharge was 1.33 kg phosphorus and 15.2 kg nitrogen per tonne shrimp.

Introduction

Although the origin of inland culture of marine shrimp in low-salinity water (2–6 ppt) has not been established with certainty, this practice initially became commercially important in Thailand in the early 1990s. Estimates of the amount of inland shrimp farming in Thailand vary, but it has been reported that 11,504 ha of ponds in 12 central Thailand provinces were devoted to this activity and that in 1998 these ponds contributed to about 30% of annual Thai shrimp production (Fast and Menavasta, 2000). Inland shrimp farming continues to be important in Thailand (Wudtisin and Boyd, 2011), and it has spread to several other countries and particularly Brazil (Nunes and Lopez, 2001), China, and the United States (Roy et al., 2010).

In Thailand, most inland shrimp farming is done in freshwater areas where brine solution from seawater evaporation ponds is mixed with freshwater (Limsuwan et al., 2002, Wudtisin and Boyd, 2011). In most other countries – including the United States – water from saline aquifers usually is the water source (Roy et al., 2010). There are over 100 countries in arid regions where surface waters and ground waters with more than 1 ppt salinity are common (Keren, 2000). Moreover, saline ground water may also occur in humid and semi-arid regions as a result of underground salt deposits, formations containing connate water of marine origin, and saltwater intrusion in coastal areas (Cook, 1997, Boyd et al., 2009). Saline ground water often has low concentrations of potassium and magnesium that negatively affect shrimp survival and growth, but it may be treated with potassium and magnesium fertilizers to counteract cation imbalances (Boyd et al., 2007, McNevin et al., 2004, Pine and Boyd, 2010, Wudtisin and Boyd, 2011) Thus, there apparently is considerable potential for expansion of inland, low-salinity shrimp aquaculture (Roy et al., 2010).

The major environmental concern expressed about inland shrimp farming is possible salination of streams, aquifers, and soils by pond discharge (Boyd et al., 2006a, Boyd et al., 2006b, Braaten and Flaherty, 2001, Pine and Boyd, 2010, Roy et al., 2010). Inland shrimp producers are aware of these concerns and may strive to lessen discharge—both to avoid salination and to conserve saline water and potassium and magnesium applied to ponds in fertilizer. In other types of pond aquaculture, there is concern about discharge of nutrients, organic matter, and suspended solids that could cause eutrophication and sedimentation in receiving water bodies. This issue has not been raised for inland shrimp farms, apparently because the potential for salination is obvious and efforts to reduce salt discharge will lessen release of other pollutants.

The purpose of this study was to evaluate effluent volume and pollutant loads at a low-salinity, inland shrimp farm in Alabama.

Section snippets

Farm and management

The study was conducted during two annual grow-out cycles – 2008 and 2010 – at a shrimp farm in southeastern Greene County, Alabama about 5 km north of Forkland on state Highway 43. This farm has 17 earthen ponds without plastic liners (Fig. 1) ranging from 0.49 to 2.02 ha in “full-pond” water surface area for a total of 21.77 ha. Average full depths vary from 1.26 m to 2.07 m (average = 1.58 m). The total water volume for the farm is 340,079 m3.

Ponds were completely filled with saline ground water (≈4 

Results

The total volumes of water on the farm at the beginning of harvest were 306,987 m3 and 302,360 m3 in 2008 and 2010, respectively. Following harvest, six ponds were empty in 2008, and seven ponds were empty in 2010. Several other ponds contained less than 75% of capacity after harvest both years. Total discharge estimated as total pond storage volume at the beginning of harvest minus water remaining after harvest was 154,826 m3 in 2008 and 174,931 m3 in 2010—50.4% and 57.9% of pond water became

Discussion

The United State Environmental Protection Agency (EPA) defined warm water concentrated aquatic animal production (CAAP) facilities as ponds, raceways, or other production units that produce over 45,455 harvest kilograms per year but excluded facilities that discharge less than 30 days/year except for excess runoff (Federal Register, 2004). The farm is a CAAP facility because it produced over 45,455 kg of shrimp each year, and because ponds were drained over a 44-day period in 2008 and a 46-day

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