MiniSipper: A NEW HIGH-CAPACITY, LONG-DURATION, AUTOMATED IN-SITU WATER SAMPLER FOR ACID MINE DRAINAGE MONITORING

Currently available automated water samplers are typically large, heavy, collect 24 one liter samples and cannot operate in freezing conditions. High-sensitivity multi-element analytical methods such as inductively coupled plasma mass spectrometry (ICP-MS) only require a few mLs of sample per analysis so large sample volumes are often unnecessary. The USGS has developed a small, light, low-cost, high-capacity, in-situ water sampler (MiniSipper) to overcome the limitations of currently available automated samplers. The MiniSipper injects 1 to 20 mL discrete or integrated water samples into a long narrow sample coil. A stabilizing reagent (HNO₃) is added and adjacent samples are separated with a N₂ gas bubble. Over 220 five mL water samples can be collected with <3% carryover. After recovery, samples are pumped out of the sample coil and analyzed by high sensitivity multi-element methods.

The in-situ design of the MiniSipper allows for easy concealment under streambed rocks and the MiniSipper has successfully collected samples under surface ice for over-winter sampling. The large number of water samples collected (>200) and long deployment duration (up to 12 months) of the MiniSipper greatly reduces field-site visits and costs. The MiniSipper is ideal for monitoring key hydrologic events which are difficult to sample or predict such as storm pulses, spring snowmelt flush, or winter low-flow periods. MiniSippers were deployed in support of Environmental Protection Agency (EPA) projects evaluating metal inputs from acid mine drainage areas in Colorado, USA. Our initial results reveal high correlations between potentially toxic metals (Al, Cu, Ni, and Zn) and specific conductivity. The large number of samples collected by the MiniSipper over the entire water year provides the requisite data to examine the robustness of conductivity-metal relationships. MiniSipper results will help guide EPA sampling strategy and help evaluate the use of real-time conductivity as a proxy for real-time trace metal concentrations. Temporally dense metal and streamflow data provide a better estimate of annual metal loading and the MiniSipper can be a key tool for monitoring TMDLs in acid mine drainage areas. A smaller MiniSipper, the Borehole MiniSipper, is under development for long-term water sampling from monitoring wells.