Trends in the quality of water in New Jersey streams, water years 1998-2007

Abstract

Trends were determined in flow-adjusted values of selected water-quality characteristics measured year-round during water years 1998–2007 (October 1, 1997, through September 30, 2007) at 70 stations on New Jersey streams. Water-quality characteristics included in the analysis are dissolved oxygen, pH, total dissolved solids, total phosphorus, total organic nitrogen plus ammonia, and dissolved nitrate plus nitrite. In addition, trend tests also were conducted on measurements of dissolved oxygen made only during the growing season, April to September. Nearly all the water-quality data analyzed were collected by the New Jersey Department of Environmental Protection and the U.S. Geological Survey as part of the New Jersey Department of Environmental Protection Ambient Surface-Water Quality Monitoring Network. Monotonic trends in flow-adjusted values of water quality were determined by use of procedures in the ESTREND computer program. A 0.05 level of significance was selected to indicate a trend. Results of tests were not reported if there were an insufficient number of measurements or insufficient number of detected concentrations, or if the results of the tests were affected by a change in data-collection methods. Trends in values of dissolved oxygen, pH, and total dissolved solids were identified using the Seasonal Kendall test. Trends or no trends in year-round concentrations of dissolved oxygen were determined for 66 stations; decreases at 4 stations and increases at 0 stations were identified. Trends or no trends in growing-season concentrations of dissolved oxygen were determined for 65 stations; decreases at 4 stations and increases at 4 stations were identified. Tests of pH values determined trends or no trends at 26 stations; decreases at 2 stations and increases at 3 stations were identified. Trends or no trends in total dissolved solids were reported for all 70 stations; decreases at 0 stations and increases at 24 stations were identified. Trends in total phosphorus, total organic nitrogen plus ammonia, and dissolved nitrate plus nitrite were identified by use of Tobit regression. Two sets of trend tests were conducted —one set with all measurements and a second set with all measurements except the most extreme outlier if one could be identified. The result of the test with all measurements is reported if the results of the two tests are equivalent. The result of the test without the outlier is reported if the results of the two tests are not equivalent. Trends or no trends in total phosphorus were determined for 69 stations. Decreases at 12 stations and increases at 5 stations were identified. Of the five stations on the Delaware River included in this study, decreases in concentration were identified at four. Trends or no trends in total organic nitrogen plus ammonia were determined for 69 stations. Decreases and increases in concentrations were identified at six and nine stations, respectively. Trends or no trends in dissolved nitrate plus nitrite were determined for 66 stations. Decreases and increases in concentration were identified at 4 and 19 stations, respectively. Introduction The Ambient Surface Water-Quality Monitoring Network (ASWQMN) program has been operated by the New Jersey Department of Environmental Protection (NJDEP) in cooperation with the U.S. Geological Survey (USGS) since 1985. Under this program, water-quality samples have been collected from streams at stations throughout the State and analyzed for water-quality characteristics. Of particular interest to the NJDEP is the quality of streams that either drain areas of targeted land-use types or are located in the downstream portions of each NJDEP water region. A total of 119 stations on nontidal streams are sampled as part of the ASWQMN; as many as 115 are sampled in any given water year (October 1 to September 30, numbered for the year in which it ends). Sampled stations include 4 on the Delaware River main stem, 7 background stations located in areas largely unaffected by human activity, and 23 watershed integrator stations with relatively large drainage basins that contain multiple land uses. There are also 43 land-use-indicator stations; the drainage basin of each is composed mostly of a single type of land use. 2 Trends in the Quality of Water in New Jersey Streams, Water Years 1998–2007 Fifteen of these stations are designated as undeveloped, 9 as agricultural, 13 as urban, and 6 as mixed land uses. In addition, 42 statewide-status stations are chosen randomly every 2 years. Each ASWGMN station is sampled four times a water year, once in each of the following periods: November to December, February to March, May to June, and August to September. Physical characteristics and concentrations of filtered and unfiltered nutrients, filtered organic carbon, and total dissolved solids are determined on samples of the water column. Unfiltered water samples are analyzed for recoverable trace elements at selected sites during February to March and August to September. Filtered samples of the water column collected during May and June are analyzed for pesticides, and bed-sediment samples collected during low-flow conditions during August and September are analyzed for nutrients and trace elements at these selected sites. These data were published annually for water years 1998 to 2005 in USGS reports titled “Water Resources Data-New Jersey.” The data for the 2006 and 2007 water years are now available on the World Wide Web and can be accessed at http://wdr.water.usgs.gov/. The USGS, in cooperation with the NJDEP, conducted a study to determine whether there have been monotonic trends in the water quality of the streams of New Jersey. The period of study is water years 1998–2007 (October 1, 1997 through September 30, 2007). A monotonic trend is an underlying change in one direction (either an increase or a decrease) in the value of a variable over the period of study; this change may occur during a short time or throughout the period of study. Water-quality data for 70 stations were analyzed. The data analyzed were collected almost entirely by the ASWQMN. The following water-quality characteristics were selected for trend analysis by the NJDEP, in cooperation with the USGS: dissolved oxygen, pH, total dissolved solids, total phosphorus, total organic nitrogen plus ammonia, and dissolved nitrate plus nitrite. Year-round measurements of each characteristic and those of dissolved oxygen made only during the growing season, April to September, were analyzed for trends. The approach was to use procedures in the ESTREND computer program (Schertz and others, 1991) to identify monotonic trends in flow-adjusted values of each waterquality characteristic at each station. Flow-adjusted values are values of water quality which have been modified to remove some of the variation in water quality due to the variation in streamflow. Purpose and Scope This report presents the results of an analysis to identify monotonic trends in selected water-quality characteristics at stations on New Jersey streams at which a sufficient number of measurements of water quality were made during water years 1998–2007. Summary statistics of the values of each waterquality characteristic at each station, and results of trend tests in flow-adjusted values, are presented in figures and tables. Results of trend tests are compared to results from previous studies of trends in the quality of New Jersey streams. Results are not reported if they were affected by changes in the methods of collection. Description of Study Area New Jersey is geologically divided into northern and southern regions by the Fall Line (fig. 1) which is marked by a series of waterfalls along river courses (Watt, 2000). The land to the north of the Fall Line is underlain by sedimentary and crystalline bedrock, is higher in elevation, and varies more in elevation than land to the south. The land to the south is underlain by gravel, sand, silt, and clay. Land use varies considerably throughout New Jersey (fig. 1). Much of the urban land lies close to the Fall Line, around New York City in the northeastern part of the State, and along the Atlantic Coast. Agricultural land tends to be present in the western half of the State, and forest is concentrated in the northwest and southeast. Previous Studies Two previous studies reported on trends in the water quality measured for the ASWQMN in streams throughout New Jersey. Both studies used the procedures in ESTREND (Schertz and others, 1991) to identify trends in water-quality values unadjusted for streamflow and in flow-adjusted values of water quality. The first report by Hay and Campbell (1990) identified changes in water-quality characteristics during two periods of record—water years 1976–86 and water years 1980–86. Trends tests were conducted on values of 48 water-quality characteristics measured at 86 water-quality stations. Results were not reported if more than a few percent of the measurements were nondetects. Trends were identified at a 0.1 level of significance. The second report by Hickman and Barringer (1999) identified changes in water-quality characteristics during water years 1986–95. Trends tests were conducted on 24 waterquality characteristics measured at 83 water-quality stations. Trends were identified at a 0.05 level of significance. In addition, Sprague and others (2009) reported trends in total phosphorus, total nitrogen, and nitrate at 20 stations on streams in New Jersey as part of their study of selected stations across the United States during the period 1993–2003. Trends in flow-adjusted concentrations and in concentrations not adjusted for flow were estimated with a parametric multiple-regression analysis by use of the program LOADEST (Runkel and others, 2004).