Perchlorate reconnaissance sampling in streams and groundwater in the Central and Southwestern U.S.
Perchlorate samples were collected using methods that ensured representative surface-water and groundwater samples. For rivers and streams this means that any water-quality variability with depth or variability across the river was integrated into the sample. For groundwater this means that stagnant water in the well casing was removed before water from the aquifer was collected. Once the sample was obtained, appropriate filtration and preservation techniques were used to ensure that water quality did not change during transit to the laboratory.
Flowing streamwater was sampled using isokinetic, depth-integrating sampling methods to obtain a representative sample. A brief summary of the sampling technique follows. Isokinetic, depth-integrating methods are designed to produce a discharge weighted (velocity-weighted) sample. Collection of isokinetic, depth-integrated samples involves using either an equal-width-increment (EWI) or equal-discharge-increment (EDI) sampling method. The EWI or EDI methods usually result in a composite sample that represents the discharge weighted concentrations of the stream cross section being sampled. Samples are collected at verticals from increments of the cross section of a stream having a specified width. The term vertical refers to that location within the increment at which the sampler is lowered and raised through the water column. EWI verticals are located at the midpoint of each width increment. EDI verticals are located at the centroid, a point within each increment at which stream discharge is equal on either side of the vertical.
The number of verticals sampled varied based on the width of the stream. Generally 10-15 vertical samples were composited at each site. However, in narrow streams it was not practical to obtain this many verticals and in very narrow streams, only one vertical sample was sampled to collect a representative sample.
When low-flow conditions (stream depth is less than unsampled zone or the stream velocity is less than 1.5 feet per second) rendered the use of an isokinetic sampler impractical, a dip sample was collected manually.
A brief summary of ground-water sample collection methods follows. Initially, a water-level measurement is made to document natural or anthropogenic stresses on the aquifer. Since water may be retained in the well casing and may not be representative of the water in the aquifer, fresh water must be drawn into the well before sampling. This is accomplished by pumping the well for a sufficient amount of time so that a minimum of three well volumes is removed from the well. When the temperature, specific conductance, and pH of the discharged water stabilized, the well was sampled by pumping water through a capsule filter with 0.45 µm pore size filter into a polyethylene bottle.
Sample preservationBoth groundwater and surface-water samples were shipped to the laboratory on ice and refrigerated until sample analysis.
Water samples were analyzed using a Dionex DX-500 Ion Chromatograph (IC) system with a GP40 pump and ED40 electrochemical detector, 2 millimeter AG16 guard and AS16 analytical columns, and 35 millimole potassium hydroxide eluent flowing at a flow rate of 0.25 milliliters per minute (mL/min). A 50-microliter injection loop was used for samples with concentrations greater than 10 μg/L, and 1,000 microliter injection loop used for lower concentrations (less than or equal to 1 μg/L). Samples were pretreated with the Ba, Ag, and H cartridges With the 2-mm column system, the signal:noise ratio at 0.2 µg/L was approximately 5.