DIURNAL VARIATIONS IN TOTAL AND DISSOLVED ARSENIC AND METAL CONCENTRATIONS, WALLKILL RIVER, NORTHWESTERN NEW JERSEY

In mid-September 2004, diurnal variations in total arsenic (As), iron (Fe), manganese (Mn) and zinc (Zn) concentrations were observed in mildly alkaline water from two sites on the Wallkill River. The sites are 2 and 10 km downstream from a mined area of the Franklin Marble, host to Zn ores and As and Mn minerals. Minimum total As concentrations (2.8 and 2.0 ppb) at both sites occurred about midnight, and maximum concentrations (3.7 and 2.6 ppb) occurred in the afternoon. Total metal concentrations increased from 1800 h to 0800 h at the upstream site and from 2000 h to 0600 h at the downstream (wetlands) site. The downstream site was resampled in September 2005. Minimum and maximum DO saturation and pH (70% and 120% and 7.77 and 8.10, respectively), typically caused primarily by photosynthesis and respiration, occurred several hours later than they did in several rivers in the western United States during summer and early fall. Streamflow increased through the night and following morning. Total metal concentrations increased during the night, as in 2004, in a cycle different from those of DO saturation, pH, and flow. Most of the total Fe (140-370 ppb) was particulate, total Zn (13-28 ppb) and total Mn (85-130 ppb) increasingly less so. Brief increases in both total As and metal concentrations tended to correspond to spikes in suspended sediment in the river. The total As cycle at the downstream site was the opposite of the total (particulate) metals cycle in 2004 and differed from it in 2005, however. The total metals cycle may be influenced by inputs such as organic acids released from aquatic plants or bacterial activities that change particle surface electrostatic properties. Concentrations of dissolved As peaked at dusk (2000 h), whereas dissolved Mn and Zn concentrations were lowest at dusk and peaked at 1000 h. Cycles in concentrations of dissolved As (as an anion) and Mn and Zn (as cations) probably differ as a result of adsorption and desorption caused by the diurnal changes in pH. In 2005, wastewater with high chloride and nutrient levels from unidentified sources appeared to have altered streamwater chemistry at the downstream wetlands site. The wastewater inputs may have delayed the diurnal cycle in temperature, the range of which (18.8-21.8 degrees C) was less than half those reported for western rivers.