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 research catchments instrumented for hydrologic monitoring - the meteorological station located in an upland clearing in the S2 research catchment (S2 MET, or South MET) and the S1 bog as part of the Spruce and Peatland Responses Under Changing Environments (SPRUCE) experiment. 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 (expected to end during 2025). The MEF is operated and maintained by the USDA Forest Service, Northern Research Station. The SPRUCE experiment is a multi-year cooperative project among scientists of the Oak Ridge National Laboratory operated by UT-Battelle, LLC and the USDA Forest Service, Northern Research Station. The SPRUCE experiment is funded by the US Department of Energy, Biological and Environmental Research Program.
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 oxygen and deuterium natural-abundance stable isotopes of water (O-18 and D).
SITE DESCRIPTION:
The overstory trees on uplands in the MEF are predominately trembling aspen (Populus tremuloides) and paper birch (Betula papyrifera). The peatlands, which are both ombrotrophic bogs, 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 the SPRUCE experiment located in the S1 bog: 5 temperature treatments (+0, +2.25, +4.5, +6.75, +9 degrees C) at ambient carbon dioxide, and the same 5 temperature treatments at elevated carbon dioxide (+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 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 catchment (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. At S2 MET, rainfall drained through about 1 m of reinforced clear vinyl tubing into a 2-L HDPE wide-mouth, graduated collection bottle. The collection bottle sits on the ground and is exposed to sunlight. At the B1, B2, and B3 collectors in S1 bog, 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 tubing to a bottle cap. The narrow adaptor relative to the wide (approx. 5 cm), flat bottle cap reduces evaporative loss from a sample bottle. In addition, collection bottles at B1, B2, and B3 were placed underneath the boardwalks (used to access the SPRUCE infrastructure in the bog) to shield the samples somewhat from sunlight and create a cooler environment near the saturated bog surface to further reduce evaporative potential and photo-exposure that may alter sample chemistry.
After March 5, 2011, the funnel at S2 MET was 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. The B1, B2, and B3 collectors were always surrounded by those types of shields.
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 (snow bucket with outside diameter = 30.5 cm, height = 22.9 cm, capacity = 15 L) was placed in a mounted bracket. Buckets were not used until autumn/winter of 2010. No frozen precipitation samples were collected at S2 MET before then.
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 1.5 m above the ground surface (S2 MET) or about 2 m above the boardwalk surface and about 2.5 m above the bog or ground surface (S1 bog). The precipitation collectors are exposed to both wet and dry deposition.
PRECIPITATION SAMPLING:
Bulk 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.
About 250 mL of water was required to complete bottle rinses and full chemistry and isotopic analyses. 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. 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 catchment (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 snow buckets were retrieved, covered, and placed inside trucks. Liquid water was needed for further processing. Oftentimes, snow melted during the 35 min drive to the Forestry Sciences Lab. 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 always 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. In S1 bog, 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 periodically 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 multiple 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 serial ID number was assigned to all aliquots of the same sample for tracking purposes in the laboratory and data reporting. Samples from S2 MET have a 6 digit integer and started with 346,298. Samples from the S1 bog have 5 digit integer starting with 82,044.
ANALYTICAL METHODS:
Water samples were analyzed for pH, specific conductivity; cation, anion, nutrient concentrations, and total organic carbon concentrations at the Forestry Science Laboratory in Grand Rapids, Minnesota.
Data values below the detection limit are reported in the data file and are not flagged. Detection limits are listed below and must be considered when using these data.
Unless otherwise noted, autosamplers were used with instruments for analysis. For each type of laboratory measurement, every tenth sample was run in duplicate followed by two reference standards.
A Mettler Toledo (Columbus, OH) DL53 Autotitrator was used to measure pH according to Standard Method 4500-H+ B (APHA 1995). A four-point calibration was performed before each batch of 15 samples. A sodium carbonate reference was run after every ten samples. Samples were only analyzed if the reference values were accurate to within 10 percent.
Conductivity was measured on a Yellow Springs Instruments (YSI; Yellow Spring, Ohio) Model 3100 meter with a YSI 3403 probe (cell constant = 1.0/cm) before March 2017 and a YSI 3253 probe (cell constant = 1.0/cm) thereafter. The meter is calibrated with a 46.7 microSiemen/cm standard, and periodically checked with 23.8, 84.0, or 150 microSiemen/cm references. Samples were manually poured into the cell to rinse the cell twice and then conductivity was measured on the third poured aliquot. Samples, the standard, and reference standards are measured at room temperature and in a laboratory maintained at 21 degree C. Conductivity values were recorded on paper. Specific conductivity (conductivity at 25 degree C) was calculated from conductivity (at 21 degree C) before values were transferred to spreadsheets.
For anion, cation, nutrient, and TOC analyses, check and reference solutions were made in volumetric flasks with deionized water (18.0 megaohm/cm). When check standards differed by more than 5% from actual values, a batch of samples was reanalyzed. When a particular sample was higher in concentration than the highest calibration standard, that sample was diluted and re-run until within the range of the calibration standards.
Anion (chloride and sulfate) concentrations were measured using suppressed conductivity detection on two different ion chromatographs: on a Dionex (Sunnyvale, CA) DX-500 before 2013 and a Thermo Scientific Dionex ICS-2100 thereafter. Samples were injected through 20 micrometer filter caps and through an IonPac AG14 pre-column and AS14 column. Standard Method 4110 B (APHA 1995) was used and the method detection limits were 0.01 mg chlorine/L for chloride and 0.02 mg sulfate/L.
Cation (calcium and magnesium) concentrations were analyzed by inductively coupled argon plasma optical emission spectroscopy (ICP-OES). Two different instruments were used: a Thermo Elemental Iris Intrepid ICP-OES before 2016 and a Thermo Scientific ICAP 7600 Duo thereafter. Standard method 3120 (APHA 1995) was used and the method detection limits were 0.05 mg calcium/L and 0.05 mg magnesium/L.
Ammonium was measured according to the Lachat QuikChem 10-107-06-1-F method on a Lachat QuickChem 8500. Ammonium is reported as the amount of nitrogen in ammonium and the method detection limit was 0.01 mg nitrogen/L.
Nitrate+nitrate was measured according to Lachat QuikChem 10-107-04-1-B on a Lachat QuickChem 8500. Nitrate was reduced to nitrite using a cadmium column and concentration is determined as the amount of nitrogen in the resulting nitrite. The method detection limit was 0.002 mg nitrogen/L. Through nitrite could not be distinguished from nitrate using this method, it has been rare in our experience to observe nitrite in MEF water samples. All samples were also analyzed by ion chromatography (data not reported here) and nitrite peaks were not visible in sample chromatograms.
Soluble reactive phosphorus was measured according to the Lachat QuikChem 10-115-01-1-B method on a Lachat QuikChem 8000 before March 2013 and Lachat QuikChem 8500 thereafter. The method detection limit was 0.001 mg phosphorus/L.
Total nitrogen (TN) concentrations were measured colorimetrically after in-line automated oxidation to nitrate. Concentrations were measured according to the Lachat QuikChem 10-107-04-1-P method on a Lachat QuickChem 8000 before 2017 and Lachat QuikChem E10-107-04-3-D method on a Lachat QuickChem 8500 thereafter. The method detection limit was 0.05 mg nitrogen/L.
Concentrations of TOC were measured by high-temperature combustion on a Shimadzu TOC-V CPH from 2008 to 2012 and a Shimadzu TOC-VCP for samples collected during 2013 onward. Concentrations were measured as total carbon minus inorganic carbon (TC-IC) before June 2010 and as non-purgeable organic carbon (NPOC) after that. NPOC was measured by high-temperature combustion (Standard Method 5310B, APHA 1995) using potassium hydrogen phthalate (KHP) for reference and check standards. Sebestyen et al. (2017) show that the TC-IC and NPOC are comparable to within 10% relative error as a measure of TOC concentration. We consider the two instruments and methods to be equivalent for our sites. The method detection limit was 0.5 mg carbon/L.
Water isotope samples were analyzed at various laboratories. Some samples that were collected from 2008-2010 were analyzed at the Center for the Environment Analytical Laboratory at Plymouth State University (analytical precision = 0.5 permil for delta D and 0.1 permil for delta O-18). Some samples from 2010-2012 were analyzed at the Integrated Watershed Hydrology and Biogeochemistry Research Facility at the University of Toronto. These two laboratories used the same instrument, a Los Gatos Research (Mountain View, California) DLT-100. Some samples from 2010 to 2013 were analyzed at the Ecosystem Laboratory at Oak Ridge National Laboratory using a Picarro Inc (Santa Clara, California) L1102-i instrument.
All water isotope samples were analyzed using laser absorption spectroscopy (Lis et al. 2008) and each laboratory used similar procedures. Unfiltered waters were injected 6-7 times with 0.5-1.2 uL/sample. Isotopic values were scaled relative to the Vienna Standard Mean Ocean Water (VSMOW)-Standard Light Antarctic Precipitation (SLAP) scale. Each laboratory used secondary standards that were calibrated to VSMOW and SLAP. Machine raw data were post-processed to account for machine drift and between-sample memory. Values for D and O-18 are reported in delta-notation in permil relative to VSMOW (Craig 1961).
Overall, relatively few samples have been analyzed for water isotopes. Aliquots for water isotope analysis are stored and will be added to the data publication as soon as samples are analyzed and the data are quality controlled/quality assessed. Analysis for the continuing project will be at USDA US Forest Service, Grand Rapids using a Los Gatos Research (Mountain View, California) T-LWIA-45-EP.
REPORTED VALUES:
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 (greater than 10 to 20 microSiemen/cm) and are reported as -9999 for that period. During 2018 and 2019, pH values were 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 for S1 samples and 2019 for S2 MET samples (when aliquots were frozen for analysis). Values are reported as -9999 prior to preservation with freezing. 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. 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 also measured on each sample, with most values below the detection limit (0.05 mg P/L). Accordingly, TP values are not reported, though, like cations, TP values were supportive in assessment of contamination.
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. In these cases, the chemistry of the entire sample was considered contaminated and all pH, specific conductivity, and concentration values for that sample are reported as -9999.
Water isotopes are not prone to the same contamination causes and issues as other solutes. Therefore, water isotopes may eventually be included even if 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:
APHA. (1995). Standard methods for the examination of water and wastewater (19th ed.). Washington, DC: American Public Health Association / American Waters Works Association / Water Environment Federation.
Craig, H. (1961). Isotopic variations in meteoric waters. Science, 133(3465), 1702-1703. https://doi:10.1126/science.133.3465.1702
Lis, G., Wassenaar, L. I., and Hendry, M. J. (2008. High-precision laser spectroscopy D/H and 18O/16O measurements of microliter natural water samples. Analytical Chemistry, 80(1), 287-293.
Sebestyen, S. D., Funke, M. M., Cotner, J., Larson, J. T., and Aspelin, N. A. (2017). Water chemistry data for studies of the biodegradability of dissolved organic matter in peatland catchments at the Marcell Experimental Forest: 2009-2011. Retrieved from: https://www.fs.usda.gov/rds/archive/Product/RDS-2017-0067/. doi:10.2737/RDS-2017-0067
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
