TRACER-BASED STUDIES OF SOIL-WATER MOVEMENT IN SEMIARID FORESTS OF NEW-MEXICO

The related issues of water movement and contaminant transport in arid and semi-arid environments have generated considerable interest and c oncern in the last few decades. Essential to understanding these issue s is knowledge of how water moves through the soils that form the uppe rmost part of the vadose zone. The use of tracers, both natural and ar tificially introduced, is proving to be an effective method for gainin g such knowledge in dry regions, where investigation by other means is difficult. In this study, natural stable-isotope and chloride tracers were used to investigate water movement in the soils of a pinon-junip er woodland and of a ponderosa pine forest on the Pajarito Plateau in northern New Mexico. The objectives were to (1) estimate and compare n ear-surface flux rates and evaluate the importance of evaporation in t he two communities, and (2) determine to what extent differences in fl ux rates and evaporation are due to differences in plant cover and/or soil hydraulic properties. The results of this study will aid in evalu ating the potential for contaminant mobility in semi-arid systems such as the Pajarito Plateau and, in addition, will increase understanding of nutrient distributions and plant water use in semi-arid environmen ts. The stable-isotope data indicate a similarity between the pinon-ju niper and ponderosa communities with respect to evaporation: in both, it is restricted mainly to the upper 10 cm of soil. Chloride profiles from the two communities, on the other hand, show a distinct differenc e with respect to downward fluxes: in the ponderosa pine forest, these fluxes (approximate to 0.02 cm year(-1)) are an order of magnitude lo wer than those in the pinon-juniper woodland (approximate to 0.2 cm ye ar(-1)), even though total precipitation is about 4 cm year(-1) higher in the ponderosa pine forest. This difference, however, appears to be related not to plant cover, but to differences in soil hydraulic prop erties. The soils of the ponderosa pine forest contain clay-rich B hor izons that appear to restrict downward movement of water through the s oil matrix, whereas the soils of the pinon-juniper community have B ho rizons much lower in clay content. The effect of differing soil proper ties on water movement suggests that contaminant distributions will va ry across the Pajarito Plateau. The data on soil water ages support th is hypothesis: they indicate that water (and, thus, contaminants) move s through the soil matrix in less than a decade in some areas, whereas in other areas, water takes hundreds of years to pass through the ent ire soil profile. (C) 1997 Elsevier Science B.V.