dissolved organic carbon

AGW02 Measurement of groundwater physical and chemical properties from wells in contrasting land uses near Kings Creek, Konza Prairie


Wells were drilled in two sites on Konza Prairie Biological Station in April, 1993 approximately 100 m from Kings Creek. The two sites are located in a grassland and an agricultural area. The grassland site (K01A) is an old field that was planted with brome sometime prior to 1976. It has not been grazed for 25 years and is burned in spring every 1-2 years. The agricultural site is currently under a mix of cultivation and restoration plots. Historically, it was cultivated from sometime between 1939 and 1950 to the present. It is approximately 1 km downstream in an area geologically similar to K01A. The soil at both sites is mapped as Reading silt loam (fine, mixed, mesic Typic Arguidolls). Samples are taken monthly by PVC bailer following removal of 2 times the well volume. Samples are analyzed with same methods as used for LTER stream water chemistry.

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To study variations in ground water chemistry between an agricultural site (A) and a grassland site (P).


Location of Sampling Stations: Fourteen wells total: Seven wells are located in an agricultural (A) site near HQ. The site is currently under a mix of cultivation and restoration plots. Historically, the site was cultivated from sometime between 1939 and 1950 to present. Seven wells are located in K01a (P). This site is an old field that was planted with brome prior to 1976. It has not been grazed and is burned in the spring every 1-2 years. Both sets of wells are approximately 100 m from Kings Creek. The two sites are approximately 1 km apart.

Frequency of Sampling: Monthly, as weather permits.

Variable Measured: Nitrate-N, ammonium-N, soluble reactive phosphate (SRP), total nitrogen (TN), total phosphorus, dissolved organic carbon (DOC).

Field Methods: Depth to water and depth to bottom of the well are measured; amount to be removed is calculated using wells volume chart and two times the volume is removed to 'flush' the well. Samples for nutrient determinations are collected from wells A-2, A-5, P-2 and P-5. At the end of a bailing session, bailers and all other equipment are cleaned with well water at Konza HQ.

Laboratory Methods: Upon return to laboratory, field workers pour each of 4 water samples into 3 separate cleaned vials and store these in the lab freezer.

One vial is analyzed for inorganic nitrogen (N) and phosphorous (P). A second vial is run for DOC. The third vial is run for TN and TP. At the completion of all analyses the vial with maximum volume is stored in the freezer as an archived sample for 10 years from date of collection.

Nitrogen and phosphorous nutrient determinations are analyzed on an OI Analytical Flow Solution IV (FSIV) instrument. Simultaneous determinations of nitrate and SRP may be analyzed from a single aliquot by use of a stream splitter. In the same manner, TN and TP are analyzed simultaneously. Ammonium determinations are performed as a single analyte. A windows based software program (WINFLOW) purchased with the FSIV allows automation of the analyzer. Regression curve information, graphic display during analyses, and calculated results provide the operator immediate information about samples. At the completion of each run, data files are electronically stored on instrument’s computer and also transferred to offsite backup data storage.

Inorganic Nitrogen (N) and Phosphorous (P) (nitrate, ammonium, and ortho- phosphate or SRP, soluble reactive phosphate) are run within one month of sample collection.

As nitrate and SRP are analyzed simultaneously, dual standards (both N and P) are utilized. Concentrations range from 0.5 to 1500 ug/L NO3 -N and 0.5 to 150 ug/L SRP. Ammonium standards range from 0.5 to 200 ug/L NH4-N. The concentrations of most well water samples are found within these ranges of concentrations. As these samples are analyzed, they are checked immediately by the Lab Research Assistant (RA) and rerun immediately if values are off. Data are entered by the Lab RA into theMaster Excel spreadsheet.

Total N and total P concentrations values are determined in a two-step process, utilizing a Total Persulfate digestion (modified from J.J. Ameel American Environmental Laboratory, October 1993) followed by N0₃ and SRP analysis on FSIV instrument. In general a series of 8 duplicated standard solutions, a digestion recovery standard (i.e. spike solution; ATP and urea), 4 spiked samples, an oxidizing reagent blank and 92 samples comprise each digestion run. Dual standards (both N03 and SRP) range in concentration from 0 to 2000 ug/L NO3 -N and 0 to 200 ug/L SRP. Pyrex screw-top digestion tubes are used for this procedure utilizing potassium persulfate as the oxidizing agent in an autoclave digestion for 55 minutes at 17 psi. A 3N sodium hydroxide reagent is used in this procedure for maintaining proper pH. Digested samples are then analyzed for NO3 -N and SRP using FSIV instrument. A digestion recovery value is calculated from digestion recovery standards and spiked samples and then applied to all samples to determine corrected TN and TP concentration values. Approximately 1 liter of cocktail solution (matches the final chemical composition of samples and reagents) is digested along with samples and used as the carrier solution for FSIV determinations.

DOC are analyzed using Schimadzu TOC-L. Five milliliters of sample are shaken and filtered through ash-free glass fiber filters into cuvettes. Samples are acidified with 0.1 mL 2N hydrochloric acid for removing dissolved CO2 during purging process prior to combustion in analyzer. Standards of 0, 1, 2 and 5 mg/L are included in each run at the beginning, middle, and end of each set of unknowns. These are used to generate regression curves for each run to calculate concentration values of samples.

1996-2011: DOC were analyzed in batches of 50 using a Schimadzu T-5000 by a trained student worker. It was possible to do approximately three batches or 150 samples per week. Through the duration of the DOC run, a continuous strip-chart will simultaneously print out after each sample is analyzed by the TOC-5000. When analysis is completed, the strip-chart is read and the mean values (MN) will be recorded onto the original data sheet, which was started in the preparation process. These mean values will be later referred to as 'peak areas'. At this point, the yymmdd, site, and mean values should be filled out and can be transferred into the TOC Data Template. These files can be located by 'Stream Nutrient on ‘Sunfish’ (J:)' in the 'ALL TOC INSTRUCTIONS, TEMPLATES, AND DATA’ folder under 'TOC Detailed Instructions'. Further detailed instructions, such as operations processes and data entry techniques are also available in this folder.

Once the template is opened, save a new file using the run date of the most recent DOC run. Fill in the 'Peak Area' cells for the DI blanks, standards, and samples. The standards are used to generate a regression curve (R2), in which the spreadsheet will automatically calculate and graph this value, as well as fill in the mg/L conversion. After the curve is developed, the concentrations of the unknowns are determined by plotting the 'peak areas' onto the regression.

The values of the areas are copied into the 'TOC Compilation' spreadsheet, located next to the TOC Data Template. The R.A. will further analyze the data and determine any outliers or unusual values for possible re-runs of the samples. Data is compiled annually, checked by lead PI, RA and archived by the IM.

Summary of All Changes:
2002: May - a bailer came untied and lodged into the bottom of A-2. Numerous attempts have been made to remove it but all have failed.

2005: May 2005 to March 2006 - unable to open P-7. Top hinge had to be permanently dismantled to gain access.

April – December 2006 – “meso” well sample added to the sampling regime for monitoring purposes.

2008: August - a large portion of the agricultural well area (A-1, 2, 3, 4 and 7) was sprayed with round-up.

2009: Summer - the metal housing around the PVC access tube of P-4 is full of water. The metal lid was not closed properly and it rained. The access tube was covered with plastic at the time; rain did not get into the well. Waiting for the water to evaporate—housing is water tight.

2009: Autumn - there is significant ground “heaving” around the bases of P-2, P-4, P-5 and P-6. The cement pad at the base of the housing has been raised up out of the ground 1 to 4 inches.

2010: March 5. Repairs have been made to the prairie wells (P-2, 3, 4, 5, 6 and 7). Dr. Gwen Macpherson recommended packing the base of the cement slab with bentonite to eliminate the possibility of overland water flow into the wells. Water from Kings Creek was used to wet down the bentonite. P-4 also has a large crack in the cement base; the cement is “pulling” away from the metal housing. Bentonite was poured into the crack and around the base.

For future reference: Bentonite for these repairs was obtained from KSU Grain Science via Dr. Keith Behnke. It is used as a binder in animal feed. Other sources are drilling operations (Associated Environmental in Manhattan) and cheap kitty litter.
August 2011: Schimadzu TOC-L installed. This analyzer automatically reads and stores DOC concentration of unknowns using a regression curve generated by known concentration standards.

January 2014 – adding samples from ColW (Columbia well) and HQW (Headquarters well) to the sampling regime. ColW is at the SE corner of the Columbia storage building. The source well for ColW covers the Columbia building, experimental stream and greenhouse (meso) north of the gas pumps. HQW is north of the Barn. The headquarters’ source well covers all HQ buildings, the corral, and over to the cabins.

December 2016 – consistent problems with the spigot and pump at ColW. Switching to the old “meso” well. We will continue calling the sample ColW.

April 2017 – a 1L bailer came untied and is now stuck in A-1. Bailer cannot be retrieved.

July 2019 – the expected depth at well P-3 was 23.0 feet. Starting in July it moved up to 22.7 feet. As of February 2020, it seems to be settled at 22.4 feet. Field data sheets have been changed.

February 2020 – problems with the spigot at HQW. We will switch to a spigot south and east of the barn; at the SE corner of the Researchers Shop and continue calling it HQW.

April 2020: AGW021 - TN data from 2001-2018 available upon request.

January 2023 – There has been a change in the depth at 3 wells; A-1, P-1, and P-3. These changes were noticed at various dates during the 2022 season.

wells  depth   new_depth
A-1    32.0    32.5
A-2    34.6    34.6
A-3    28.2    28.2
A-4    20.2    20.2
A-5    35.7    35.7
A-6    20.1    20.1
A-7    28.0    28.0
P-1    23.4    21.1
P-2    27.5    27.4
P-3    23.0    22.4
P-4    18.5    18.5
P-5    28.0    28.0
P-6    17.8    17.8
P-7    22.8    22.8

For additional metadata information see: http://lter.konza.ksu.edu/sites/default/files/DC.pdf
For additional methods information see:: http://lter.konza.ksu.edu/sites/default/files/MM.pdf

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