Use of Chloride Concentration to Assess Conservative and Non-Conservative Properties of Everglades Surface-Water Constituents
Michael G. Waldon, and Paul McCormick
DOI Everglades Program Team, Boynton Beach, FL
Joint Conference on the Science and Restoration of the Greater Everglades (GEER) and Florida Bay Ecosystem, Palm Harbor, Florida, April 2003.
The Arthur R. Marshall Loxahatchee National Wildlife Refuge includes water conservation area number 1 (WCA-1), one of three WCAs that were built to maintain water storage and flood control as well as provide a protected refuge for the remnant Everglades ecosystem. In the 1950s and 1960s, WCA-1 was completely surrounded by perimeter canals and hydrologically isolated from its watershed by levees. Stormwater runoff primarily from the Everglades Agricultural Area, but also from urban sources, is pumped into the perimeter canal where it may flow to discharge structures or mix into the rainwater-dominated interior wetland.
Relative to the rainwater-dominated interior of WCA-1, the pumped stormwater has elevated concentrations of a number of constituents including chloride, calcium, sulfate, conductivity, total nitrogen, and total phosphorus. A pollutant-impacted fringe of marsh has developed between the relatively pristine interior and the perimeter canals. This impacted fringe marsh extends over a significant fraction of the total refuge area (Richardson et al. 1990).
This presentation will document a preliminary graphical assessment of the transformations of constituents introduced in the pumped stormwater passing down the perimeter canals and across the fringe marsh of WCA-1. In estuarine systems, mixing plots (also called concentration-salinity diagrams) are a widely used method to identify presence of sources and sinks of constituents during seaward transport and mixing (Middelburg and Nieuwenhuize 2001; Sharp et al. 1982; Steen et al. 2002; Uncles et al. 1998). Mixing plots graph a water quality parameter against salinity. For a purely conservative parameter, observations should fall on a straight line drawn from the parameter value and salinity characteristic of the freshwater inflow to value and salinity characteristic of the seaward boundary. If observations primarily fall above this mixing-line there is evidence that a source is present. If observations fall primarily below the mixing line there is evidence of some loss or sink mechanism for the parameter. A wide scatter above and below the mixing line may indicate presence of both a source and sink. An analogous approach is applied here to study the behavior of chemical properties during mixing of pumped stormwater with water originating as rainwater within the refuge interior. Here, chloride concentration rather than salinity is used to estimate the fraction of each water sample originating as pumped inflows to the refuge.
Data collected by the South Florida Water Management District from a continuing transect monitoring study were analyzed (McCormick et al. 2000). Sites analyzed include two in the perimeter canal, designated X0 and Z0, and 3 marsh sites, designated X4 Y4 and Z4, located respectively 4.4, 3.2, and 3.1 Km from the canal. Water quality parameters characteristic of pumped inflow were identified as the median value of canal observations when canal conductivity was above median. Similarly, parameters characteristic of the interior were estimated as the median for the 3 interior sites when marsh conductivity was below median. Plots were classified (Table 1) by visual inspection as being characteristic of a conservative, a source, or a sink (C + and – respectively).
Table 1. Characterization of water qualityparameters.
The results summarized in Table 1 are mostly unsurprising. Loss of many constituents from interior marsh water could be attributed to biologically driven processes such as precipitation (calcium), microbial reduction (sulfate and inorganic nitrogen forms), and uptake (inorganic nitrogen and phosphorus). The existence of a source of total phosphorus in the perimeter canals is troubling. This source persisted after Stormwater Treatment Area-1W began operation, and appears at times when canal stage is below, as well as above marsh ground elevation. It is conjectured that the internal canal sources for total phosphorus and calcium are related to groundwater discharge and further that the phosphorus source results from internal loading caused by advection of re-mineralized sediment pore water phosphorus into the water column. Recent studies (Daroub et al. 2002) have documented a large pool of phosphorus in the highly organic sediments that have been deposited in the perimeter canals since their construction.
This study illustrates the utility of monitoring and analyzing a suite of water quality parameters rather than focusing on single parameters in isolation. Results from this study bear on fundamental questions in Everglades restoration including period required for recovery, possible unintended deleterious impacts of restoration projects, and STA performance. Continued research efforts will attempt to quantify the processes identified here, including estimation of the rate of canal internal loading of total phosphorus. These efforts may include statistical analysis, and steady state or dynamic water-quality modeling.
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Corresponding author: Michael G. Waldon, Arthur R. Marshall Loxahatchee National Wildlife Refuge, 10216 Lee Road, Boynton Beach, FL 33437, Phone: 561-732-3684, Fax 561-732-3867, email@example.com