United States
Environmental Protection
Agency
Environmental Monitoring
Systems Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S9-87/006 May 1987
Project Summary
Comparison of Weekly and
Daily Wet Deposition
Sampling Results
L. E. Topol, M. Lev-On, A. K. Pollack, and T. J. Permutt
The objectives of this project were to
evaluate the changes in chemical com-
position that may occur when precipi-
tation is stored up to seven days in col-
lectors, and to assess the significance
of the variability of those changes from
site-to-site and from season-to-season.
In a one-year field program, two pairs of
identical wet deposition samplers were
deployed at three sites; for each pre-
designated pair, either daily samples or
weekly samples were collected. Two
sites also had a pair of collocated
weighing bucket rain gauges. Common
procedures were used at all three field
sites and all samples were analyzed at
the same laboratory.
The data obtained were used to de-
termine differences between daily and
weekly sampling for (a) collection effi-
ciency (by reference to the same rain
gauge) for each site and by precipita-
tion type, (b) precision of daily and
weekly monitoring, (c) concentration
bias (expressed as the relative differ-
ence between derived weekly and
measured weekly concentrations) over-
all and by season, and (d) deposition
bias (defined as the difference between
the deposition calculated from the daily
measurements and that from the
weekly measurements).
This Project Summary was devel-
oped by EPA's Environmental Monitor-
ing Systems Laboratory, Research Tri-
angle Park, NC, to announce key
findings of the research project that is
fully documented in a separate report
of the same title (see Project Report or-
dering information at back).
Introduction
Precipitation sampling networks in
the United States generally collect ei-
ther daily (UAPSP, MAP3S) or weekly
(NADP/NTN) samples. Under a weekly
schedule, samples can remain in the
collector under ambient conditions for
up to seven days, possibly resulting in
more chemical changes than might
occur if the samples remain in collectors
for at most 24 hours. When comparing
chemical composition and trends calcu-
lated from weekly and daily deposition
networks, it is important to know the
occurrence and magnitude of such
changes.
To determine the importance of such
chemical changes, a collocated sam-
pling study was performed at three sites
of the Utility Acid Precipitation Study
Program (UAPSP) network from Octo-
ber 1983 to October 1984. The sites—
Uvalda, Georgia; Lancaster, Kansas;
Underhill, Vermont—were selected to
represent the southeastern, central
(west of the Mississippi River), and
northeastern regions of the United
States.
The three UAPSP monitoring sites
were equipped with four identical pre-
cipitation samplers, two collecting daily
samples, and two, weekly samples. This
was done to allow precision data to be
calculated for both sampling schedules
and also to allow a comparison to be
made of the chemical composition for
the daily and weekly samples. All analy-
ses were performed in the same labora-
tory and except for the sampling sched-
ule, all procedures were identical. In
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addition, two sites had collocated rain
gauges so that the precision of precipi-
tation depth and deposition could also
be determined.
Procedures
Field Procedures
All operators were trained in site op-
erations based on the U.S. EPA
"Operation and Maintenance Manual
for Precipitation Measurement Sys-
tems" and the UAPSP "Field Operator
Instruction Manual." Daily site visits
were made at about 0900 to check the
equipment and remove any precipita-
tion collected by the daily samplers.
Daily samples were weighed and a
portion removed from each for pH and
conductivity measurements. These
measurements were made about an
hour after sample removal from the col-
lector to allow for temperature equili-
bration with the standard solutions re-
quired for meter calibration and the pH
quality control check. When the sample
was frozen or contained snow, it was
allowed to thaw before an aliquot was
removed for field analysis. If the sample
remaining after pH and conductivity
measurement was greater than 20 g, a
500 ml sample (or whole sample if less
than 500 ml) was transferred to a clean,
labeled, plastic bottle and sealed; the
rest of the sample was discarded. If pH
and conductivity measurement were
less than 20 g, the sample was dis-
carded.
Weekly samples were removed on
Monday from the Georgia and Vermont
sites and on Tuesday at the Kansas site.
The same procedures were followed
with the weekly samples as with the
daily samples, except that the weekly
samples were shipped on the day of
pickup and were not refrigerated prior
to shipment. However, both daily and
weekly samples stored at 4°C, were
shipped together to the analytical labo-
ratory. If no event occurred within a
week, the buckets were rinsed with
deionized water to remove any dust that
may have deposited, and then reused.
The rinse water was shipped to the cen-
tral laboratory for analysis as a dynamic
blank.
Each sample was identified by site,
sampler, date, and weight. To prevent
confusion of the sample identity, since
there were four samplers at a site, the
sampler buckets were inscribed with an
identification number. The information
was recorded both on the sample bottle
and on a data sheet. Also listed on the
data sheet were the site values of the pH
and conductivity, the rain gauge precip-
itation reading(s) in inches per event,
the number of lid openings, and any
pertinent observations by the site oper-
ator.
Analytical Procedures
When a sample arrived at the labora-
tory, the temperature of the box interior
was measured. Each sample was
logged in, and the temperature and any
pertinent codes were recorded on the
data sheets. Approximately every twen-
tieth sample was marked for analysis in
duplicate as a quality control measure.
All the samples were assigned consecu-
tive numbers. The chemist performing
the analysis in the laboratory was not
aware of either the collection schedule
or the identity of the collocated sam-
ples.
All samples were analyzed without fil-
tration. Filtration has the advantage of
removing both bacteria, which can de-
grade nitrogen and phosphorus com-
pounds as well as organic acids, and
soil particles, which are generally basic
and react with hydrogen ion. However,
filtration has the disadvantage of being
a source of contamination. To minimize
degradation, the samples were kept
cold after removal from the collector,
during shipment, and before and after
analysis at the laboratory. In addition,
portions of the sample removed for
analysis were decanted to eliminate
sedimented particles.
Conductivity and pH were determined
within one week of sample receipt,
using standard laboratory pH and con-
ductivity meters. S04, NO3 and Cl were
determined by ion chromatography and
NH4 was determined by a colorimetric
method. Na, K, Ca, and Mg were deter-
mined by an atomic absorption tech-
nique. Generally, all samples were ana-
lyzed for their ion concentration within
six weeks of receipt.
Results
A statistical analysis of the data using
nonparametric techniques determined
the following:
• The collection efficiencies were
highest for rain and about equal for
snow and mixed precipitation; i.e.,
a value of about 1.1 for rain and
0.75 for mixed precipitation and
snow. Generally, small differences
were observed between weekly
and daily sampling and among
sites for the same precipitation
type.
• The precision of ionic concentra- A
tion determinations was compare- '
ble or better for daily than for
weekly samples. Overall measure-
ment precision for both sampling
protocols (with the exception of
potassium) was less than 20 per-
cent of the ionic concentration.
• The measured weekly concentra-
tions were, in most cases, approxi-
mately 10 percent higher than the
weekly concentrations derived
from daily samples.
• The differences between weekly
and composited daily concentra-
tions varied by constituents, but
generally were less than 10 percent
of the mean concentration. Also,
the individual ion concentration bi-
ases were of similar magnitudes
and direction among the sites ex-
cept for acidity and sodium.
• There were seasonal variations in
the concentration bias between
daily composited and weekly sam-
ples.
• Differences in paired rain gauge
depths at the two sites equipped
with collocated rain gauges were
greatest for snow samples. Overall,
the paired rain gauges at the Ver-
mont site measured significantly
different precipitation amounts;
the differences were greatest in the
winter quarter.
• Precision of daily depositions (cal-
culated from the two daily collec-
tors and the two rain gauges) was
better at the Kansas site (for most
ions the precision was 5 to 15 per-
cent). At the Vermont site, where
the rain gauge differences were
larger, the precision of the ion dep-
ositions was 10 to 30 percent.
• Calculated deposition amounts
varied by season at each site. Peak
sulfate deposition occurred in the
spring at the Kansas and Georgia
sites, and in the summer at the Ver-
mont site. Peak acidity deposition
values occurred in the summer at
the Vermont and Georgia sites, and
in the spring at the Kansas site.
Conclusions
This study determined that for most
ions, concentrations measured by daily
sampling were more precise than those
from weekly sampling. In other words,
daily sampling provided narrower con-
fidence intervals for concentration
means, and could allow earlier detec-
tion of significant trends in precipitation '
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composition. Although weekly meas-
ured concentrations were generally
higher than weekly concentrations
derived from daily samples, the sea-
sonal variability in the results obtained
from daily versus weekly sampling was
not consistent either in magnitude or di-
rection. This indicated that daily and
weekly sample collection can provide
consistent spatial distribution results (at
least for the three sites studied), but that
temporal distributions obtained from
weekly samples might be incompatible
with those obtained from daily samples.
Therefore, for a network that changes
from daily to weekly sampling, the bias
in the results could interfere with trend
analysis.
L Topol and M. Lev-On are with Combustion Engineering. Newbury, CA 91320;
and A. Pollack and T. Permutt are with Systems Applications Inc., San Rafael,
CA 94903.
W. J. Mitchell is the EPA Project Officer (see below).
The complete report, entitled "Comparison of Weekly and Daily Wet Deposition
Sampling Results," (Order No. PB 87-168 837/AS; Cost: $18.95, subject
to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Monitoring Systems Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
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