For a routine water-quality sample where near-surface water is representative of the water mass, a water sample can be collected by directly immersing the container beneath the water surface to a depth of 1 ft. A bucket can be used to collect a sample if the mixed surface layer is very shallow or accessible only from a bridge. If a bucket is used, extreme care should be taken to avoid contaminating the sample with debris from the rope and bridge. Care must be taken also to rinse the bucket between stations. In slow-moving rivers, reservoirs, and estuaries, the depth of the mixed layer can be determined from field measurements by locating the thermocline or an abrupt change in specific conductance. In tidally-influenced water bodies, the mixed surface layer is defined as that part of the water column from the surface to the depth at which the specific conductance is 6,000 ตS/cm greater than the specific conductance at the surface. For mixed surface-layer samples (depth greater than 1 ft), pre-rinse one of the following sampling devices at least once with native water before using: submersible pump tube, Kemmerer, or Van Dorn. A minimum volume of 3 L should be collected from each site. Sample containers do not have to be rinsed with site water. Care should be taken at all times during sample collection, handling, and transport to prevent exposure of the sample to direct sunlight. Information on sample processing, preservation, and holding times for the grab sampling method, including information on dissolved metals in water, is listed in table 3.