Environmental Engineering Reference
In-Depth Information
from slightly less than 0 (negative flux is move-
ment from the river to the subsurface) to more
than 500 L/m
2
d.
Figure 8.9
clearly illustrates the
variability in exchange rates across a streambed
and highlights the limitation of using single
point estimates, such as obtained from a seep-
age meter measurement or from a heat-trans-
port approach, to estimate those exchange
rates. Point estimates of exchange rates are very
useful for describing variability in those rates
over space and time, but, as demonstrated in
this study, single point estimates may not be
adequate for describing average flux across a
stream transect or reach.
0
1
2
3
4
5
6
7
8
Example: Middle Rio Grande,
Albuquerque, New Mexico
Vertical temperature profiles and groundwater-
level altitudes were measured in piezometers
installed along the banks of the Rio Grande at
four locations in central New Mexico (Bartolino
and Niswonger,
1999
). The data were used
to calibrate VS2DH simulations of water and
heat flow between the river and the underly-
ing aquifer. The objective of the study was to
generate estimates of recharge to the aquifer.
The Rio Grande is an important regional source
of water for agricultural and municipal uses.
Temperatures were measured in 0.15 m depth
increments on seven dates between September
1996 and August 1998 (
Fig ure 8.10
). The wide
seasonal temperature envelope extends unusu-
ally deep to depths greater than the 15 m maxi-
mum depth of measurement.
One-dimensional models of water and heat
transport were constructed for each of the four
locations to simulate the seasonal trends in
groundwater levels and temperatures. Thermal
and hydraulic properties, with the exception
of hydraulic conductivity, were obtained from
the literature on the basis of lithology and bulk
density of soil samples. The bottom boundary of
the simulated column consisted of fixed heads
and temperatures that were determined from
measured values (interpolated between meas-
urement times). The PEST parameter estimation
program was used to determine values of ver-
tical hydraulic conductivity and top boundary
9
10
11
12
13
14
15
16
0369 2 5 8 1 4 7
Temperature (°C)
09 SEPT 96
18 OCT 96
03 DEC 96
22 JAN 97
22 APR 97
02 JUNE 98
12 AUG 98
Figure 8.10
Groundwater temperature profiles adjacent
to the Rio Grande at a site near Albuquerque, New Mexico
(after Bartolino and Niswonger,
1999
).
temperatures that produced the best agree-
ment between measured and simulated water
levels and temperatures. Final results showed
that simulated temperatures differed from
measured values by 0 to 2.1°C (most differences
were less than 0.5°C). Calculated drainage rates,
averaged over the study period, were between
2 × 10
-7
and 4 × 10
-7
m/s.
