Analysis of mid- and high-stage conditions for the Peconic River at the eastern boundary of Brookhaven National Laboratory, Suffolk County, New York

Brookhaven National Laboratory (BNL) has historically discharged sewage treatment plant (STP) effluent to the Peconic River, which runs through the BNL site in Suffolk County, N.Y. This effluent discharge has averaged about 700,000 gallons per day (about 1.1 cubic feet per second [ft³/s]) since 1962 and led to contamination of streambed sediments by radioactive and hazardous constituents. Large sections of the stream channel near BNL are dry during periods of relatively low water-table altitude referred to as low-stage conditions. During mid-stage conditions, the water table intersects the streambed and base flow commences and increases as the water table rises to the tops of the streambanks. Areas adjacent to the stream become flooded during high-stage conditions as the water table rises above the streambanks. Information on the long-term (1943-2003) percentages of time that discharges at two nearby streamflow-gaging stations exceeded thresholds associated with mid- and high-stage conditions is needed to provide a range of estimates of the prevalence and seasonal variability of these conditions during the same years for streamflow-gaging station HQ on the Peconic River at the eastern boundary of BNL. Analysis and correlation of discharge data from the three streamflow-gaging stations—BNL’s station HQ and the U.S. Geological Survey stations on the Peconic River at Riverhead, N.Y., and Carmans River at Yaphank, N.Y.—were performed to extend the 1995-2003 period of record for station HQ.

Low-stage conditions occur when there is no flow at station HQ and, therefore, the start-of-flow for the Peconic River is downstream of BNL property. Mid-stage conditions occur when there is flow at station HQ but its daily mean value does not exceed 4.2 ft³/s; high-stage conditions occur when this discharge exceeds 4.2 ft³/s. Daily mean streamflows at station HQ were associated with low-stage conditions most of the time during 1995-2003 for all flow durations. Low-stage conditions predominated during January, March, and July through December of these years, whereas mid-stage conditions prevailed during parts of February and April through June. Mid-stage conditions generally appeared throughout the year during 1995-2003, except for mid-October, during which only low-stage conditions were observed. High-stage conditions were attained the least amount of time for all flow durations, and appeared only during parts of March through July and December of these years.

The percentages of time during 1943-2003 that daily mean streamflows at the Riverhead and Yaphank stations were associated with low-, mid-, and high-stage conditions provide a range of estimates of the amounts of time that these conditions occurred during these years at station HQ. Daily mean streamflows were associated with low-stage conditions most of the time during 1943-2003 for durations of 30 and 60 days; with mid-stage conditions most of the time for durations of 1, 3, and 7 days; and with either of these conditions for a duration of 14 days. High-stage conditions were attained the least amount of time during these years for all durations, except perhaps that of 1 day, for which low-stage conditions could have occurred the least amount of time. Mid-stage conditions predominated during January through early March, June through early July, and late November through December of these years. These conditions typically appeared throughout the year during 1943-2003, and occurred most often during late February. High-stage conditions also generally appeared throughout the year, except perhaps for a few days during early September of these years, and occurred most often during April. These results indicate that streamflows observed during 1943-2003 at the Riverhead and Yaphank stations—used to estimate a longer record for station HQ—were considerably higher than those observed during 1995-2003 at the three stations, and provide information that can be used in future studies to better understand the long-term capacity of streams such as the Peconic River near BNL to supply continuous flow, flood adjacent low-lying areas, and sustain aquatic habitats.