We monitored water chemistry at five freshwater reservoirs and their tributaries from 2013 - 2023 at varying frequencies. The five drinking water reservoirs are: Beaverdam Reservoir (Vinton, Virginia), Carvins Cove Reservoir (Roanoke, Virginia), Falling Creek Reservoir (Vinton, Virginia), Gatewood Reservoir (Pulaski, Virginia), and Spring Hollow Reservoir (Salem, Virginia). Beaverdam, Carvins Cove, Falling Creek, and Spring Hollow Reservoirs are owned and operated by the Western Virginia Water Authority as primary or secondary drinking water sources for Roanoke, Virginia, and Gatewood Reservoir is a drinking water source for the town of Pulaski, Virginia. The dataset consists of depth profiles of water chemistry samples measured at the deepest site of each reservoir adjacent to the dam. Additional water chemistry samples were collected at a gauged weir on Falling Creek Reservoir's primary inflow tributary, as well as surface samples at multiple upstream and inflow sites in Falling Creek Reservoir 2014-2023 and Beaverdam Reservoir in 2019 and 2020. One upstream site at BVR was sampled at depth in 2022. Inflow sites at Carvins Cove Reservoir were sampled from 2020 - 2023. The water column samples were collected approximately fortnightly from March-April, weekly from May-October, and monthly from November-February at Falling Creek Reservoir and Beaverdam Reservoir, approximately fortnightly from May-August in most years at Carvins Cove Reservoir, and approximately fortnightly from 2014-2016 in Gatewood and Spring Hollow Reservoirs, though sampling frequency and duration varied among reservoirs and years. Water chemistry variables measured include: Total nitrogen and phosphorus, nitrate, ammonium, soluble reactive phosphorus, dissolved carbon, dissolved organic carbon, dissolved inorganic carbon, and total dissolved nitrogen.
In the methods, we describe collection methods, chemical analysis including equipment, QAQC methods, and additional references.
SAMPLE COLLECTION AND EQUIPMENT
Most sampling occurred between the hours of 9:00 and 15:00 eastern standard time with daylight savings observed; however, some sampling occurred outside of these hours, including some overnight sampling. For more information about nighttime sampling, see Doubek et. al. (2018). Starting in 2018 to present, exact sampling times were included in the DateTime column starting in 2018 and are indicated in the DateTime column with a Flag_DateTime value of 0. Prior to 2018, if exact times were not recorded during sample collection, time was set to 12:00 and Flag_DateTime was set to 1.
Total nitrogen (TN) and total phosphorus (TP) unfiltered water samples were collected at specified depths for each reservoir using a 4L Van Dorn water sampler (Wildco, Yulee, Florida, USA). Samples were stored in acid-washed 125 mL polypropylene bottles and frozen within 12 hours. Total nutrient samples were generally analyzed within one year of collection date.
Soluble reactive phosphorus (SRP), nitrate (NO3), ammonium (NH4), dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), and total dissolved nitrogen (DN) water samples were collected at specified depths for each reservoir using a Van Dorn water sampler and were filtered with a 0.7 um glass fiber filter (Sterlitech GF/F) before being stored in acid-washed 125 mL polypropylene bottles. Soluble/dissolved nutrient and carbon samples were generally analyzed within six months of collection date.
CHEMICAL ANALYSES AND EQUIPMENT
TN and TP samples were digested with alkaline persulfate (Patton and Kryskalla 2003) and then analyzed colorimetrically using flow injection analysis (APHA 2012). TN was analyzed using the cadmium reduction method (APHA 2012) and TP was analyzed using the ascorbic acid method (Murphy and Riley 1962) on a Lachat Instruments XYZ Autosampler ASX 520 Series and QuikChem Series 8500 (Lachat ASX 520 Series, Lachat Instruments, Loveland, Colorado, USA).
SRP, NO3, and NH4 samples were analyzed colorimetrically using flow injection analysis (APHA 2012). SRP was analyzed using the ascorbic acid method (Murphy and Riley 1962), NO3 was analyzed using the cadmium reduction method, where nitrate is reduced to nitrite (APHA 2012), and NH4 was analyzed using the Berthelot Reaction method (Solorzano 1969, APHA 2012) with a common modification as to the source of the hypochlorite ion, as described by Zhang et al. (1997), on a Lachat Instruments XYZ Autosampler ASX 520 Series and QuikChem Series 8500 (Lachat ASX 520 Series, Lachat Instruments, Loveland, Colorado, USA).
DOC was analyzed using the persulfate catalytic method (Brenton and Arnett 1993) on a TOCA 1010 from OI Analytical from 2013-2016 (OI Analytical 1010 Total Organic Carbon Analyzer with 1051 autosampler, College Station, TX USA) and on a Vario TOC Cube from Elementar from 2016-2023 (vario TOC cube, Elementar Analysensysteme GmbH, Hanau, Germany). Carbon in samples is oxidized to carbon dioxide (CO2) either by reaction with acid or by catalyzed combustion at 850 degrees C. The resulting carbon dioxide is detected by nondispersive infrared (NDIR) spectrometry. This method allowed for the measurement of total dissolved carbon, dissolved organic carbon, and dissolved inorganic carbon. A modified version of this method was used to measure dissolved organic carbon through the measurement of non-purgeable organic carbon (NPOC) in streams that had high inorganic carbon (IC) values. Organic carbon was still measured following the method above but the sample was first acidified and purged with air zero gas to remove inorganic carbon fractions. Samples measured with this method were flagged with an 8 in the Flag_DOC column.
DN samples were combusted using the Vario TOC Cube from Elemantar at 850 degrees C. Total bound nitrogen in the combustion product is converted to nitrogen monoxide (NO) by oxidative pyrolysis and then reacts with an electrolyte in the electrochemical cell, producing a measurable current to calculate total dissolved nitrogen (organic and inorganic together).
For more details on instrument transitions and analytical chemistry methods performed during the study period, see Supporting Information Text 2 in Carey et al. (2022).
METHOD DETECTION LIMITS
Starting in 2020, we changed our Method Detection Limit (MDL) calculations and adopted those which are described in the USEPA second revision (USEPA 2020). The new analytical MDL calculations rely on data obtained from independent digestions over multiple runs, rather than that from a single day's run, to more accurately capture instrument performance and variability throughout the year.
QAQC SCRIPTS
Two scripts are used to QAQC and compile published data. 'QAQC_chemistry_2015_2023.R' is used to QAQC samples collected in 2023 that were analyzed between summer 2023 and winter 2024. This script applies data flags for MDLs and sets negative values to zero. 'Chemistry_inspection_2013_2023.Rmd' is used to combine 2023 data with the previous years' publication, review all data via visual inspection, and apply any additional QAQC needed for previous years' data.
DATA FLAGS
Flags for date time values were: 0 = exact time, 1 = no time recorded and set to noon
Flags for all chemistry values were the following with the exception of flag 8, which was only applied to DOC values: 1 = sample not taken, 2 = instrument malfunction, 3 = sample below detection, 4 = negative value set to zero, 5 = demonic intrusion, 6 = non-standard method, 7 = sample run multiple times and values averaged, 8 = sample run using NPOC method due to high IC values, 9 = suspect sample
We note that measurements with multiple flags are coded as a multiple-digit number (e.g., a flag of '43' indicates there was 4 = negative value set to zero and 3 = sample below detection). No delimiter was added to separate flag codes in those columns. For data with a '74' flag, data were set to 0 before averaging to get the final sample value.
PLEASE NOTE
When pulling the water chemistry data file via EDI's API in R, we recommend using the function "read.csv" instead of "read_csv". The function "read_csv" identifies the columns as "logical" instead of "double" due to >100 NA's at the beginning of the dataset. This is avoided when using the function "read.csv".
ADDITIONAL NOTES
Multiple whole-ecosystem experiments have been conducted at Falling Creek Reservoir, including intermittent operation of hypolimnetic oxygenation (HOx) and pulsed epilimnetic mixing (EM) engineering systems. We encourage you to contact the lead author of the data package for more information.
REFERENCES
APHA. 2012. Standard methods for the examination of water and wastewater. 22nd edn. Washington, DC: American Public Health Association, American Water Works Association, Water Environment Federation.
Brenton R, Arnett T. 1993. Method of analysis by the U.S. Geological Survey National Water Quality Laboratory - Determination of dissolved organic carbon by UV-promoted persulfate oxidation and infrared spectrometry. Denver, CO: U.S. Geological Survey.
Carey, C. C., Hanson, P. C., Thomas, R. Q., Gerling, A. B., Hounshell, A. G., Lewis, A. S. L., Lofton, M. E., McClure, R. P., Wander, H. L., Woelmer, W. M., Niederlehner, B. R., & Schreiber, M. E. (2022). Anoxia decreases the magnitude of the carbon, nitrogen, and phosphorus sink in freshwaters. Global Change Biology, 28, 4861–4881. https://doi.org/10.1111/gcb.16228
Patton CJ, Kryskalla JR. 2003. Methods of Analysis by the U.S. Geological Survey National Water Quality Laboratory--Evaluation of Alkaline Persulfate Digestion as an Alternative to Kjeldahl Digestion for Determination of Total and Dissolved Nitrogen and Phosphorus in Water. Denver, CO: U.S. Geological Survey.
Revesz KM, Doctor DH. 2014. Automated determination of the stable carbon isotopic composition (d13C) of total dissolved inorganic carbon (DIC) and total nonpurgeable dissolved organic carbon (DOC) in aqueous samples: RSIL lab codes 1851 and 1852: U.S. Geological Survey Techniques and Methods, book 10, chap. C20, 38 p., http:// dx.doi.org/10.3133/tm10C20.
Solorzano, L. (1969) Determination of Ammonia in Natural Waters by the Phenolhypochlorite Method. Limnology and Oceanography, 14, 799-801.
USEPA. 2004. RSKSOP-175 STANDARD OPERATING PROCEDURE Sample Preparation and Calculations for Dissolved Gas Analysis in Water Samples Using a GC Headspace Equilibration Technique, Revision No.2 http://www.epa.gov/region1/info/testmethods/pdfs/RSKsop175v2.pdf Retrieved 20APR2015.
USEPA, E. 2020. Definition and procedure for the determination of the method detection limit, revision 2. Washington DC, USA: EPA; 2016.
Zhang, J.Z., Orter, P., Fisher, Ch. J. and Moore, L.D. 1997. Determination of ammonia in estuarine and coastal waters by gas segmented flow colorimetric analysis. Methods for determination of chemical substances in marine and estuarine environmental matrices. 2nd ed. EPA/7664-41-7.
