A MINI DRIVEPOINT SAMPLER FOR MEASURING PORE-WATER SOLUTE CONCENTRATIONS IN THE HYPORHEIC ZONE OF SAND-BOTTOM STREAMS

A new method for collecting pore-water samples in sand and gravel stre ambeds is presented. We developed a mini drivepoint solution sampling (MINIPOINT) technique to collect pore-water samples at 2.5-cm vertical resolution. The sampler consisted of six small-diameter stainless ste el drivepoints arranged in a 10-cm-diameter circular array. In a simpl e procedure, the sampler was installed in the streambed to preset driv epoint depths of 2.5, 5.0, 7.5, 10.0, 12.5, and 15.0 cm. Sampler perfo rmance was evaluated in the Shingobee River, Minnesota, and Final Cree k, Arizona, by measuring the vertical gradient of chloride concentrati on in pore water beneath the streambed that was established by the uni nterrupted injection to the stream for 3 d. Pore-water samples were wi thdrawn from all drivepoints simultaneously. In the first evaluation, the vertical chloride gradient was unchanged at withdrawal rates betwe en 0.3 and 4.0 ml min(-1) but was disturbed at higher rates. In the se cond evaluation, up to 70 ml of pore water was withdrawn from each dri vepoint at a withdrawal rate of 2.5 ml min(-1) without disturbing the vertical chloride gradient. Background concentrations of other solutes were also determined with MINIPOINT sampling. Steep vertical gradient s were present for biologically reactive solutes such as DO, NH4+, NO3 -, and dissolved organic C in the top 20 cm of the streambed. These de tailed solute profiles in the hyporheic zone could not have been deter mined without a method for close interval vertical sampling that does not disturb natural hydrologic mixing between stream water and groundw ater.