Persistance and movement of diquat and the effectiveness of limnobarriers after curlyleaf pondweed treatment in Crystal Lake, Connecticut

Diquat concentrations in Crystal Lake, Middletown, CT, USA were monitored after diquat was applied in late April to the southern half of the lake to control curlyleaf pondweed (Potamogeton crispus L.). Limnobarriers were installed around a small island and a stretch of shoreline to protect Vasey's pondweed (Potamogeton vaseyi J. W. Robbins), which is an aquatic plant listed as ' 'threatened' ' in Connecticut. Surface and bottom water were analyzed for diquat from treated and untreated lake sites, inside and outside limnobarrier sites, and downstream until 24 d after treatment (DAT). Mean diquat concentrations in the treated surface lake sites peaked at 327 g mu L super(-1) 0.2 DAT (5 h) and were no longer detectable 13 DAT. At 5 DAT, the treated and untreated lake surface sites had similar mean diquat concentrations of 32 and 38 mu g L super(-1), respectively. Diquat concentrations in the treated lake surface sites gradually declined until they were no longer detectable 13 DAT. Vertical movement of diquat into the bottom water in the treated lake sites was limited, with a peak concentration of 7 mu g L super(-1) at 7 DAT. Diquat in the lake's outlet stream followed a pattern similar to the untreated surface water, but the concentration peaked at only 26 mu g L super(-1) at 3 DAT. Inside the limnobarriers, diquat concentrations were significantly reduced but not eliminated. Diquat concentrations peaked inside the island and shore limnobarriers 2 DAT at 75 and 39 mu g L super(-1) respectively, and became nondetectable 13 DAT. Diquat movement into the limnobarriers could have been facilitated by imperfect seals where sections connect. The Vasey's pondweed inside the limnobarriers did not appear impacted.