Hm. Valett et al., PARENT LITHOLOGY, SURFACE-GROUNDWATER EXCHANGE, AND NITRATE RETENTIONIN HEADWATER STREAMS, Limnology and oceanography, 41(2), 1996, pp. 333-345
We address the ecological ramifications of variation in hydrologic int
eraction between streams and alluvial aquifers in catchments with allu
vium derived from parent materials of contrasting geologic composition
. We present a conceptual model in which solute retention in streams r
esults from hydrologic retention (increased water residence time resul
ting from surface-groundwater exchange), biological nutrient cycling,
and chemical processes. Solute injection experiments were done in stud
y catchments comprised of sandstone-siltstone (site 1), volcanic tuff
(site 2), and granite-gneiss (site 3). Distribution of an injected con
servative tracer (Br) illustrated that rate and extent of surface-wate
r penetration into the alluvial aquifer increased across study catchme
nts as was predicted from increasing alluvial hydraulic conductivity.
Concurrently, groundwater inputs at baseflow represented between 13 an
d 57% of aboveground discharge at upstream transects, indicating bidir
ectional hydrologic exchange along the study reaches. N:P ratios in su
rface water ranged from 4 to 16, suggesting strong biotic demand for i
norganic N. Coinjection of NaBr and NaNO3 revealed longest nitrate upt
ake length (S-w) at site 1, intermediate S-w at site 2, and shortest u
ptake length at site 3. Modeling of transient hydrologic solute storag
e revealed that S-w correlated with hydraulic storage, suggesting an i
mportant role for subsurface processes in total nitrate retention.