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Quantification of LoadsLoad is the mass of a constituent transported downstream. For chemically conservative constituents, loads are additive as inflows contribute their load to the load in the receiving stream. The effect of loads on instream concentrations is dependent on the volume of water transporting the mass. For comparative purposes, loads are commonly expressed as a rate of transport in terms of mass per unit time (for example, mg/s for an instantaneous load; kilograms/year for an annual load). Instantaneous load, as used in this synoptic study, is calculated as the product of concentration and streamflow at the time of sampling. Instantaneous loads were calculated for the 25 mainstem and 31 surface-inflow sites for calcium, magnesium, sodium, and sulfate as well as for dissolved and total recoverable aluminum, cadmium, copper, iron, lead, manganese, and zinc (table 7). Table 7. Instantaneous loads of major ions and metals in the Daisy Creek and Stillwater River drainage, Montana, August 26, 1999 (click here for pdf file) (click here for xls file) Downstream load profiles graphically illustrate the spatial distribution of loads at many locations along the mainstem and can be used to identify locations of important sources and sinks that affect the constituent load. Stream segments where constituent loads are added to or removed from the water column can be identified by comparing three different loading profiles: the instream load in the mainstem, the cumulative surface-inflow load, and the cumulative instream load. The profile of instream load represents the instantaneous loads (table 7) measured at each mainstem sampling site. Instream loads were calculated by multiplying streamflow by constituent concentration. Downstream changes in instream loads are the net result of inputs from the sampled surface inflows and any unsampled subsurface inflow, as well as any loss of load caused by streamflow loss, formation of colloids and subsequent deposition on the streambed, or other geochemical reactions (Broshears and others, 1995; Kimball and others, 1994). The instream load profile is important because it defines the net effect of all inputs to the stream and losses in the stream. Partitioning of metals between the dissolved and colloidal phases can be assessed by comparing the dissolved and total-recoverable instream loads. If these loads are equal, then the load is entirely dissolved. Subtracting the dissolved load from the total-recoverable load gives the instream particulate load, which is assumed to be primarily colloidal. The cumulative surface-inflow load at a site is the sum of all surface-inflow loads downstream to that point and is determined by summing surface-inflow loads in the downstream direction. The profile of cumulative surface-inflow load does not include loads contributed by subsurface inflows, which are calculated as the difference between instream loads and surface-inflow loads. Surface-inflow loads were calculated from the streamflow data in table 5 and the concentration data in table 6. The inflow load for a constituent was assumed to be zero if the constituent concentration was less than the minimum reporting level. A third load profile that is useful for load analysis is the cumulative instream load. This load is calculated for each mainstem site by adding all individual increases in load between successive upstream mainstem sites. The profile of cumulative instream load indicates the entire load that entered the stream from surface and subsurface sources, disregarding any downstream loss from chemical or physical processes. The amount of subsurface loading to the mainstem can be estimated by subtracting the cumulative surface-inflow load from the cumulative instream load. The amount of dissolved load that has been converted to colloidal load in the mainstem can be calculated by subtracting the instream dissolved load from the cumulative instream dissolved load. |
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