Several naturally occurring tracers of water in natural systems (for example, stable isotopes of oxygen and hydrogen) are being widely used to determine the flow paths that water takes through a catchment. These chart the course of water from when it lands on the soils surface as rain or snow until it flows out of the catchment as streamflow. Tracing of hydrological flow path, in conjunction with geochemical studies of how water interacts with rocks and soils, has led to new understanding of the hydrochemistry of upland forested catchments.
Although a large fraction of precipitation that reached the forest floor infiltrates into the very permeable soils, studies have disproved the once-prevalent notion that water slowly percolates through soils and rocks. Instead, preferred flow paths along old root channels, dessication cracks, and other heterogeneities in the soils transmit water and solute rapidly both vertically and downslope. This rapid movement profoundly affects the chemical reactions in the soils, influencing, for example, how "acid rain" affects the chemical composition of soil and stream water. Major findings from recent work are that downslope transport occurs along preferred paths in the shallow, normally saturated portion of the soil; and that riparian areas (wetlands immediately adjacent to the stream channel) play an extraordinarily important role in catchment hydrochemistry.
Use of stable isotopes to determine whether stormflow in streams consists of "old" or "new" water continues to support the contention that the bulk of water appearing in streams is old. (That is water that has resided in the catchment for a long enough time-at least a month-to acquire a relatively uniform isotopic signature that distinguishes it from immediate rainfall.) Old water tends to be "heavier" (to contain a higher ratio of O18 to O16, for example) than immediate precipitation, because evapotranspiration tends to fractionate water in this direction. Nevertheless, measurements show rapid flow along preferred paths. They also show that water follows relatively shallow flow paths during large runoff-events.
Evidence comes from direct hydrological and chemical measurements over a range of scales and from indirect inference based on naturally occurring and induced tracers. Continued creative field research will be required to elucidate the mechanisms that allow the infiltrated water to mix rapidly with the old water in the soil, and to understand the mechanisms of rapid downslope transport.
The fact that streamwater appears to be a mixture of old and new implies that its overall chemical composition should be deliverable as a simple mixture of the end member-waters. (Some recent work suggest that three types of waters mix to comprise stream water, but the essential concept is the same.) For many chemical species this mixing concept has been found valid, but for some species, particularly those that are quite active biologically, it has been found that hydrological-biogeochemical interactions in the riparian zone have a major impact on altering the chemical composition of old water from upslope as it flows out into the stream. The exact mechanisms responsible for the chemical changes are not yet well-established.
George can be reached at (415) 853-8300
Originally published in EOS Reprinted with permission.
