Water Resources Research
Supporting Information for
Quantifying watershed-scale groundwater loading and in-stream fate of nitrate using highfrequency water quality data
Matthew P. Miller*1, Anthony J. Tesoriero2, Paul D. Capel3, Brian A. Pellerin4, Kenneth E. Hyer5,
Douglas A. Burns6
1U.S.
Geological Survey, 2329 Orton Circle, Salt Lake City, UT 84119 USA
Geological Survey, 2130 SW 5th Ave, Portland, OR 97201 USA
3U.S. Geological Survey, 500 Pillsbury Dr., SE, Minneapolis, MN 55455 USA
4U.S. Geological Survey, 6000 J St., Sacramento, CA 95819 USA
5U.S. Geological Survey, 1730 E. Parham Rd., Richmond, VA 23228 USA
6U.S. Geological Survey, 425 Jordan Rd., Troy, NY 12180 USA
2U.S.
Contents of this file
Figure S1
Text S2
1
a.
b.
c.
Figure S1. Trilinear diagrams showing the distributions of major ions during high BFI (> 0.8)
conditions at (a) the Potomac River (n=8), (b) Smith Creek (n=9), and (c) Difficult Run (n=3).
Carbonate and bicarbonate data were only available for 2 dates at Smith Creek and were not
analyzed on any dates at Difficult Run. While limited by the number of available samples,
especially at Difficult Run, the lack of separation among seasons in these plots suggests that it
is unlikely that there are shifts in groundwater flow paths to the streams among seasons.
2
S2. Sensitivity of k Estimates to Time-Variable Runoff End-Member Concentrations.
It is likely that runoff end-member concentrations are time-variable. Because low BFI
values occur during high flow conditions, when the influence of groundwater-discharged nitrate
and in-stream retention/loss is expected to be minimal, the concentrations measured in the
stream during these times are likely similar to the runoff end-member concentrations. In turn,
time-variable (e.g. seasonal) runoff end-member concentrations could be defined as the
average of in-stream concentrations during low BFI days during each season (or for a different
temporal designation).
To demonstrate how time-variable runoff end-member concentrations could be
estimated at sites with low BFI values during all seasons, we calculated the average in-stream
nitrate concentrations during the 10% of days with the lowest BFI values during each season at
each site (Table S1). The seasonally-variable end-member concentrations were used to
estimate [NO3]in and k values at each site following the same approach applied to the static
end-member concentrations. The percent difference between the annual average k values
calculated using the seasonally-variable and static runoff end-member concentrations was 2%
at the Potomac River, 20% at Smith Creek, and 26% at Difficult Run.
Accurate characterization of time-variable runoff end-member concentrations would
require substantial data at low BFI values (say <0.1) throughout the year. Unfortunately, such
data are not available at the three study sites presented here (see Figure 4), thereby requiring
the use of the seasonal values presented in Table S1 for the time-variable runoff end-member
example application. These seasonal end-member concentration estimates were, in some
cases, generated using concentration data on days with high BFI values (occasionally > 0.5; See
Figure 4). At such high BFI values, in-stream concentrations are substantially influenced by
groundwater-discharged nitrate and possibly in-stream retention/loss of nitrate. It is for this
reason that static end-member concentrations were used in the main text of the manuscript,
and the time-variable runoff end-member concentrations are presented as an example in the
supplementary information. Nonetheless, these results suggest that at these sites the effects
of the use of static runoff end-member concentrations on resultant k values may be small, but a
wider range of sites is needed to evaluate this assumption more broadly.
Table S1. Seasonally-variable runoff end-member concentration estimates (mg/L as N) and the
percent difference between k values calculated using the static and seasonally-variable endmember concentrations.
Potomac River
Smith Creek
Difficult Run
Winter
1.26
1.93
0.98
Spring
0.94
1.98
1.13
Tempe
Summer
0.76
2.29
1.01
Fall
1.11
1.75
0.71
Difference between annual average k values
calculated using static and seasonally-variable runoff
end-member concentrations (%)
2
20
26
3