Lake Worth Bottom Sediments—A Chronicle of Water-Quality Changes in Western Fort Worth, Texas, 1914–2001

FS_2004-3077.fm Page 1 Thursday, July 29, 2004 1:42 PM In cooperation with the U.S. Air Force Lake Worth Bottom Sediments—A Chronicle of Water-Quality Changes in Western Fort Worth, Texas, 1914–2001 Fish-Consumption Advisory Issued Because of for Toxic Substances and Disease Registry (2001) priority list of haz- Elevated PCBs ardous substances are hydrophobic. In spring 2000, the Texas Department of Health issued a fish- Chemical analysis of sediment cores is one method that can be consumption advisory for Lake Worth, Tex., because of elevated used to determine trends in HOCs such as PCBs. As sediments accu- concentrations of polychlorinated biphenyls (PCBs) in fish (Texas mulate in lakes and reservoirs, they generate a partial historical record Department of Health, 2000). In response to the advisory and in coop- of water quality. This fact sheet describes the collection of sediment eration with the U.S. Air Force, the U.S. Geological Survey (USGS) cores, age-dating methods, and historical trends in PCBs in Lake collected 21 surficial samples and three deeper gravity core samples Worth sediments. The fact sheet also describes the spatial distribution from the sediment deposited at the bottom of Lake Worth. The purpose of PCBs in surficial sediments and concludes with objectives for the of that study was to assess the spatial distribution and historical trends second phase of data collection and the approach that will be used to of selected hydrophobic contaminants, including PCBs, and to deter- achieve these objectives. The USGS published a comprehensive report mine, to the extent possible, sources of selected metals and hydropho- on the first phase of the study (Harwell and others, 2003). bic organic contaminants (HOCs) to Lake Worth. Hydrophobic Lake Worth is a reservoir on the West Fork Trinity River on the (literally “water fearing”) contaminants tend to chemically adsorb to western edge of Fort Worth in Tarrant County. In 1914, the City of soils and sediments. Fifteen of the top 20 contaminants on the Agency Fort Worth completed the reservoir to serve as a municipal water supply. Lake Worth has a surface area of 13.2 97˚29' 97˚28' 97˚27' 97˚26' 97˚25' square kilometers and a storage capacity of 47 mil- lion cubic meters. The drainage area to the reservoir 32˚50' is 5,350 square kilometers (Ruddy and Hitt, 1990). The surrounding area to the south and east is pri- Study area marily urban, and the Fort area to the north and Worth TEXAS northwest is mostly 32˚49' residential. Collection of LOCATION MAP Sediment Cores Surficial sedi- Silver Creek 0 0.5 KILOMETER 32˚48' LWT.UBLWT.UB ment samples (top 2 0 0.25 0.5 MILE 820 centimeters) were col- IH– Dam LWT.DMLWT.DM EXPLANATION lected at 21 sites using Lake Worth Live Oak Creek Area enlarged a box corer. The sites in figure 5 Woods were distributed Inlet LLWT.DMWT.DM throughout the lake LWT.WDLWT.WD Box-core sampling 32˚47' White Meandering site/gravity-core (fig. 1) to include areas Naval Air Settlement Road Creek sampling site and Station along the shoreline West Fork identifier adjacent to Air Force Carswell Trinity River Landfill Plant 4 (AFP4) and No. 3 Air Force Field Box-core sampling Plant 4 site Naval Air Station-Joint Landfill Building 181 No. 4 Reserve Base Carswell 32˚46' Base modified from U.S. Geological Survey Field, the mouths of Digital orthophoto quarter-quadrangle, major tributaries, Universal Transverse Mercator projection, Zone 14, Datum NAD 1983, 1-foot resolution areas near the dam that are influenced by all Figure 1. Location of study area and approximate sampling locations in Lake Worth, Fort Worth, Texas, 2000– sources of contaminants 2001. U.S. Department of the Interior USGS Fact Sheet 2004–3077 U.S. Geological Survey 1 July 2004 (a) (b) subsampled for chemical analysis (fig. 2c). Box-core samples were analyzed for the same constituents as gravity-core samples except for cesium-137 (137Cs), which was used in age dating the gravity cores. Results of all chemical analyses are presented in Harwell and others (2003). Age-Dating Methods The pre-reservoir boundary (dated as 1914, when the dam was completed), the sampling date in 2000, and profiles of radioactive 137Cs were used in age dating the gravity cores. In reservoir sediment (c) cores, sediment deposited after the lake formed is easy to distinguish from pre-reservoir soils because pre-reservoir soils are relatively dry and can contain sand and root hairs. Because 137Cs sorbs strongly to fine-grained sediments, it is useful for dating sediments exposed to atmospheric fallout, such as those found in lakes (Durrance, 1986). Beginning in about 1952, large-scale nuclear weapons testing released detectable amounts of 137Cs to the atmosphere. Concentrations of 137Cs in the atmosphere peaked between 1963 and 1964. Profiles of 137Cs activity in gravity cores can provide two date markers in reservoirs built before about 1950—the first occurrence between 1952 and 1953 and the peak in 1964. Other chemical indicators also can be used to corroborate age assignments made using 137Cs; these include peaks in DDT and lead concentrations. Total DDT (the sum of DDT and its breakdown products) concentrations tend to peak in the early-to-mid-1960s, coincident with peak use of DDT in the United States (Van Metre and Callender, 1997). Lead concentrations in urban lake and reservoir sediment cores consistently peaked in the mid- 1970s coinciding with the switch from leaded to unleaded gasoline (Callender and Van Metre, 1997). All of these indicators were used to Figure 2. U.S. Geological Survey personnel (a) describing a assign dates to core samples and to investigate the reasonableness of gravity core, (b) placing gravity core in core extrusion stand, date assignments. Ages for samples at depth intervals between known and (c) slicing a subsample from a gravity core (photographs depth-date markers were assigned on the basis of a mass accumulation by Barbara J. Mahler, U.S. Geological Survey). rate (MAR) because MARs automatically adjust for compaction in a core. The 137Cs activity profiles in each of the three gravity cores to the lake, and areas in the upper part of the lake not affected by AFP4 indicate different sedimentation rates for each of the sites (fig. 3). and Carswell Field and less affected by urbanization. Core LWT.UB has an unusual 137Cs profile with nondetections in At three sites where surficial sediment samples were collected, gravity LWT.UB–Upstream from IH–820 LWT.WD–Woods Inlet LWT.DM–Downstream from IH–820 bridge bridge (near dam) cores were collected to investigate his- 0 torical trends in selected contaminants. 1964 One gravity core was collected in the 20 upper part of the lake upstream from the 1932 1964 40 IH–820 bridge (LWT.UB, fig. 1) to represent historical trends in a reference 60 1932 site upstream from AFP4 and much of the urban influence to the lake. A sec- 80 1964 ond gravity core was collected in 100 Woods Inlet (LWT.WD, fig. 1) near the 120 mouth of Meandering Road Creek to 1914 1914 represent a site influenced by runoff 140 from AFP4. A third gravity core was CORE DEPTH, IN CENTIMETERS collected in the lower part of the lake 160 near the dam (LWT.DM, fig. 1) to rep- 1932 180 resent a site influenced by all inputs of 1914 contaminants to the lake. 200 -0.1 0 0.1 0.2 0.3 0.4 0.5 0 0.5 1.0 1.5 2.0 2.5 3.0 -0.5 0 0.5 1.0 1.5 2.0 2.5 3.0 One gravity core from each site CESIUM-137 ACTIVITY, CESIUM-137 ACTIVITY, CESIUM-137 ACTIVITY, IN PICOCURIES PER GRAM IN PICOCURIES PER GRAM IN PICOCURIES PER GRAM was split lengthwise and described EXPLANATION visually on site (fig. 2a). A second core Detection Concentration less than reporting limit from each site was extruded vertically Figure 3. Cesium-137 profiles in gravity-core samples from Lake Worth, Fort Worth, Texas, (fig. 2b), and discrete intervals were 2000–2001. 2 97˚29' 97˚28' 97˚27' 97˚26' 97˚25' 0 0.5 KILOMETER 0 0.25 0.5 MILE 32˚50' EXPLANATION 12 Box-core sampling LWT.UB–Upstream from IH–820 bridge site and total PCB 2010 concentration, in ND micrograms per 1990 kilogram—Non- detection (ND) is 32˚49' 1970 less than reporting level, which varies YEAR from less than 15 to 1950 less than 75 micro- ND grams per kilogram. 1930 Concentrations in ND gravity core shown 1910 0 102030405060 in graph TOTAL PCB CONCENTRATION, IN MICROGRAMS PER KILOGRAM 32˚48' ND ND concentrations from urban lakes peaked at the height of their use in the mid-to-late 9 ND ND ND 1960s, then decreased follow- 10 12 ND 12 ing the restrictions imposed in LWT.WD–Woods Inlet ND ND 2010 20 ND 9 3 15 1971 (Van Metre, Callender, LWT.DM–Downstream from IH–820 bridge (near dam) 1990 2010 and Fuller, 1997; Van Metre 32˚47' 140 and others, 1998). Because of 1970 1990 variable numbers and positions of chlorine atoms within their YEAR 1950 1970 structure, 209 individual PCB YEAR 1930 1950 compounds, or congeners, are possible. PCBs were marketed 1910 1930 32˚46' 0 10 102030405060 20 30 40 50 60 as Aroclors, mixtures of many TOTALTOTAL PCB PCB CONCENTRATION,CONCENTRATION, congeners. The analytical ININ MICROGRAMS MICROGRAMS PER KILOGRAM KILOGRAM 1910 0102030405060 method used for this report TOTAL PCB CONCENTRATION, IN MICROGRAMS PER KILOGRAM quantifies PCBs as one of three common Aroclors (1242, 1254, Figure 4.

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