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.