This data set reports the event-based chemistry of precipitation water
that was collected at the Marcell Experimental Forest (MEF) in Itasca
County, Minnesota. The data come from sites in two peatlands
instrumented for hydrologic monitoring - the meteorological station
located in an upland clearing in the S2 research catchement and the
Spruce and Peatland Responses Under Climatic and Environmental Change
(SPRUCE) experiment located in the S1 bog. Sample collection and
analyses started during June of 2008 at the S2 site and is ongoing.
Sample collection and analyses started during December of 2013 at the
SPRUCE S1 sites and will continue for the duration of the experiment.
The MEF is operated and maintained by the USDA Forest Service,
Northern Research Station. The SPRUCE experiment is a multi-year
cooperative interaction among scientists of the Oak Ridge National
Laboratory operated by UT-Battelle, LLC and the USDA Forest Service,
Northern Research Station.
Samples were analyzed for pH, specific conductivity, nutrient
concentrations (ammonium, nitrate, soluble reactive phosphorus, total
nitrogen), cation concentrations (calcium, magnesium), anion
concentrations (chloride, sulfate), total organic carbon
concentrations, and natural-abundance stable isotopes of water
(d18O-H2O and dD-H2O).
SITE DESCRIPTION:
The overstory trees on uplands in the MEF are predominately trembling
aspen (Populus tremuloides) and paper birch (Betula papyrifera). The
peatlands are vegetated with plants ranging from Sphagnum to low
shrubs to mature black spruce (Picea mariana) and tamarack (Larix
laricina) trees.
There are 10 experimental plots in SPRUCE experiment located in the S1
bog: 5 temperature treatments (+0, +2.25, +4.5, +6.75, +9 degrees C)
at ambient CO2, and the same 5 temperature treatments at elevated CO2
(+500 ppm). While bulk deposition sampling occurs under ambient
conditions outside of the SPRUCE experimental enclosures, these data
span the pre- and post-treatment periods when enclosed plots were
exposed to warming and elevated carbon dioxide (CO2) within the SPRUCE
experiment.
BULK DEPOSITION SAMPLERS:
Precipitation samples were collected year round on an event-basis from
one collector located in an upland clearing at the meteorological
station in the S2 research catchement (S2-MET) and three individual
collectors (B1, B2, B3) located at the end of three boardwalks that
are used to access the S1 bog and SPRUCE plots. Samples from the three
collectors in S1 were either composited or individually analyzed,
depending on the total volume of water available relative to the
amount needed to complete all laboratory analyses. Both individual
(B1, B2, B3) and composite (S1) samples from the three SPRUCE
collectors in S1 were analyzed when precipitation amounts were
sufficient. Funnel/bottle collectors were used to collect rainfall and
buckets were used to collect snowfall.
When rainfall was expected, typically from March or April through
October or November, a 20.3 cm (8 inch) diameter high density
polyethylene (HDPE) funnel was used to capture rainfall. Rainfall
drained through about 2 m of reinforced clear vinyl tubing into a 2-L
HDPE wide-mouth, graduated collection bottle. A narrow (0.95 cm / 3/8
inch) adaptor was used to connect the tubing to a bottle cap. The
narrow adaptor relative to the wide (approx. 5 cm), flat bottle cap
reduces evaporative loss from the sample bottle. In addition,
collection bottles were placed underneath the boardwalks (used to
access the SPRUCE infrastructure in the bog) to shield the samples
somewhat from sunlight and create a somewhat cooler environment near
the saturated bog surface to further reduce evaporative potential and
photo-exposure that may alter sample chemistry. Funnels were
surrounded by a shield with a spiked upper surface to discourage birds
from perching on and defecating into funnels (a common occurrence when
shields are not used). The rim of the funnel is at about the tops of
the spikes.
Funnel/bottle collectors were not appropriate for solid precipitation
collection. Solid precipitation would collect and remain in funnels
increasing the likelihood of evaporation and evaporative enrichment of
samples during exposure to the atmosphere. When snowfall or mixed
precipitation (i.e., some combination of rain, sleet, hail, ice, or
snow) was expected, a Nalgene Large Cylindrical HDPE Container
(outside diameter = 30.5 cm, height = 22.9 cm, capacity = 15 L) was
placed in a permanently mounted bracket.
During transitions from above-freezing to below-freezing conditions
when precipitation could be either liquid or solid, both types of
samples could be placed side-by-side and the choice of which sample to
retain for chemical analysis could be made upon collection. When both
collector types were used, the funnel/bottle samples were always
preferred due to the expected decreased likelihood of evaporation.
Collector openings are about 2 m above the boardwalk surface and about
2.5 m above the bog or ground surface. The precipitation collectors
are exposed to both wet and dry deposition.
PRECIPITATION SAMPLING:
Precipitation was collected on an event-basis, rather than a fixed
interval, to minimize exposure of samples to evaporation, sunlight,
excessive heat, or freezing if precipitation was collected as a
liquid. Samples may have accumulated over successive, closely-spaced
precipitation events or been collected mid-way through a long-duration
precipitation event. In any case, the time between sample collection
and retrieval was intended to be as minimal as possible, with sampling
typically within 12 to 24 h of the end of a precipitation event.
Precipitation events that ended after business hours, during weekends
(Friday afternoon through Monday morning), or holidays were typically
collected the next business day between 7 AM and 4 PM. The date/time
reflects when the sample was retrieved, not when the precipitation
event occurred or ended.
An individual sample was taken from each of the three funnel/bottle
collectors in the S1 bog (B1, B2, B3) when there was sufficient volume
of liquid water in each bottle. About 250 mL of water was required to
complete bottle rinses and full chemistry and isotopic analyses. When
less than 250 mL was available from each individual funnel/bottle in
the S1 bog, all water or an equal proportion from each bottle was
composited, usually into one of the three 2-L bottles, and labelled
with location S1. When there was more than 500 mL from each individual
funnel/bottle in the S1 bog, a sample was collected from each
collector, plus approx. 200 mL from each collector was composited from
all three collectors in a separate sample. Precipitation collected in
the S2 catchement (S2-MET) was always analyzed individually. Samples
were placed in the dark in iced-coolers for transport to the Forestry
Sciences Laboratory in Grand Rapids where they were then processed,
stored, and analyzed.
Frozen or mixed precipitation samples in Nalgene containers (snow
buckets) were retrieved and placed inside trucks. Oftentimes, snow
melted during the 35 min drive to the Forestry Sciences Lab, which was
needed for further processing. Sometimes snow or ice samples were
placed on laboratory counters to melt rapidly, or left in a
refrigerator to melt more slowly (for example, if retrieved late in
the day and processing would not occur until the next day). Although
volumes of snow and melted water were not measured (before 2018), if
there appeared to not be enough sample in a single snow bucket to
complete all analyses, melted water in the individual collectors from
the S1 bog was composited like liquid water samples from funnel/bottle
collectors. During and after 2018, melted precipitation water from
large events was measured with a graduated cylinder.
Whenever volumes were sufficient, an aliquot was saved from liquid or
melted precipitation water in the B1, B2, and B3 collectors from large
events and composited from each bucket. In addition, individual
buckets were sampled.
Containers with insects or other obvious contamination were not saved
for analysis or compositing. When possible, water from samples with
undecomposed plant parts or pollen was salvaged by decanting the water
from the particulates. If one field sampling container was not usable
due to contaminants, only two of three individual funnel/bottle or
snow bucket containers were composited. Some precipitation events were
not saved for chemistry analysis due to long holding times in field
collection containers or an insufficient total volume even when
composited. For the funnel/bottle collector, the bottles were replaced
with new acid cleaned bottles after each field visit whether a sample
was collected or not. The tubing was peridocally acid washed or
replaced if necessary. The snow buckets were also replaced with acid
cleaned buckets after each field visit.
At the time of collection, date/time of retrieval, sample location,
sample volume (liquid samples only), and associated notes were
recorded on field data sheets. Unfiltered water was decanted from the
2-L individual funnels/bottles or snow buckets into storage
containers: a 250 mL low density polyethylene (LDPE) bottle for pH,
specific conductivity, ion, and nutrient analyses (refrigerated),
After 2015, a separate 60-mL aliquot was saved in an HDPE bottle
solely for nutrient analysis (frozen); a 20 to 40-mL amber glass vial
for total organic carbon analysis (refrigerated); and a 16-mL
scintillation vial with a Polyseal cap for liquid water isotope
analysis (stored at room temperature). Sample bottles for ion and
nutrient chemistry were triple rinsed with precipitation water before
filling. Scintillation vials for water isotope samples were completely
filled, with no headspace or bubbles. A unique, consecutive serial ID
number (5 digit integer starting with 82,044 and currently greater
than 91,000) was assigned to all aliquots of the same sample for
tracking purposes in the laboratory and data reporting.
ANALYTICAL METHODS:
Water samples were analyzed for pH, specific conductivity; cation,
anion, nutrient concentrations, and total organic carbon
concentrations; and liquid water isotopes at the Forestry Science
Laboratory in Grand Rapids, MN. Please refer to the included methods
and detection limits table (labMethods_detectionLimits.pdf) for a
description of analytical methods, instruments, and detection limits.
Data values below the detection limit are reported in the data file
and are not flagged.
To document precipitation that was sampled, we include sample info
(laboratory ID, sample name, and date/time) for all collected samples.
Sometimes chemistry values are assigned -9999 for individual solutes
or for all analytes (i.e., pH, specific conductivity, and solute
concentrations), which may have resulted from insufficient sample
volume to complete all analyses, contamination that affected
individual solutes or suites of analytes that were simultaneously
measured on a single instrument for a particular sample, or
contamination that affected all solutes for a particular sample.
Samples pending analysis are also assigned -9999.
Chloride and pH are reported as -9999 for much of 2015 and 2016 when
there was evidence of contamination of those two analytes from
inadequate rinsing of hydrochloric acid (HCl) after acid washing of
funnels, tubing, and collection bottles used for field sampling.
Specific conductivity values were also abnormally high (10 to 20
microSiemen/cm) and are reported as -9999 for that period. During 2018
and 2019, pH values are 1 to 2 pH units below the expected range
(approx. 4-6, Turk 1983) and we do not report pH values from that
entire period. Concentrations of nitrate and ammonium are only
reported onward from 2015 (when aliquots were frozen for analysis).
Values are reported as -9999 prior to 2015. Other cations (aluminum,
iron, manganese, potassium, silicon, sodium, and strontium) were also
measured on each sample, but values were rarely above detection
limits, except when a sample was contaminated. Accordingly, these
values are not reported.
Though field notes were effective for identification of samples that
were contaminated by insects and debris (mostly leaves or needles from
the adjacent overstory forest canopy), many cation values with
concentrations that were unrealistically high for precipitation were
also indicative of particular samples that were contaminated. The
chemistry of the entire sample was considered contaminated and all pH,
specific conductivity, and concentration values for that sample are
reported as -9999. The detection limits were: 0.01 mg/L for aluminum,
0.05 mg/L for iron and silicon, 0.01 mg/L for manganese and strontium,
0.5 mg/L for potassium, and 0.1 mg/L for sodium. Total phosphorus (TP)
was measured on each sample, with most values below the detection
limit (0.05 mg/L). Accordingly, TP values are not reported.
Water isotopes are slated for analysis and will be added to the data
publication as soon as the data are quality controlled/quality
assessed. Water isotopes are not prone to the same contamination
causes and issues as other solutes. Therefore, water isotopes may
eventually be included when pH, specific conductivity and
concentrations values are reported as -9999.
MARCELL EXPERIMENTAL FOREST sites and data collection are described in
further detail in:
Sebestyen, S.D., C. Dorrance, D.M. Olson, E.S. Verry, R.K. Kolka, A.E.
Elling, and R. Kyllander (2011). Chapter 2: Long-Term Monitoring Sites
and Trends at the Marcell Experimental Forest. In Randall K. Kolka,
Stephen D. Sebestyen, Elon S. Verry, and Kenneth N. Brooks (Ed.).
Peatland Biogeochemistry and Watershed Hydrology at the Marcell
Experimental Forest (pp 15-71). CRC Press, Boca Raton, FL.
https://www.fs.usda.gov/treesearch/pubs/37979.
SPRUCE Project Website with project plans and additional information:
http://mnspruce.ornl.gov/
REFERENCES:
Turk, J. T. (1983) An evaluation of trends in the acidity of
precipitation and the related acidification of surface water in North
America, USGS Water Supply Paper. https://doi.org/10.3133/wsp2249
