TITLE: USDA LTAR Common Experiment measurement: Best practices for collection, handling, and analyses of water quality samples
AUTHORS: Oliva Pisani, Richard Lizotte, Kristen S. Veum, John L. Kovar, Stephen K. Hamilton, Robert W. Malone

[DESCRIPTION]
Following best practices during sample collection and analysis will produce data of known and defensible quality. This protocol details the best practices for the collection of water samples from croplands and for the following water quality protocols:

USDA LTAR Common Experiment measurement: Dissolved nitrate (NO3-) concentration
USDA LTAR Common Experiment measurement: Dissolved ammonia (NH3) concentration
USDA LTAR Common Experiment measurement: Total nitrogen (TN) and total dissolved nitrogen (TDN) concentration
USDA LTAR Common Experiment measurement: Total phosphorus (TP) and total dissolved phosphorus (TDP) concentration
USDA LTAR Common Experiment measurement: Total suspended solids (TSS)

[STEPS]
SECTION: Sample collection and handling for the analysis of primary water quality metrics
1. Measure the primary water quality metrics in water leaving croplands from surface runoff, subsurface flow or drain tile, and leaching or percolating water below the rooting zone. 

 

SECTION: Sample collection and handling for the analysis of primary water quality metrics
2. Collect the surface runoff and drain tile samples manually as single discrete grab samples or using automatic samplers. 
SECTION: Sample collection and handling for the analysis of primary water quality metrics
3. Collect grab samples at a specific site for a limited duration (seconds to minutes) and to represent a “snapshot” of the measured constituent. 
SECTION: Sample collection and handling for the analysis of primary water quality metrics
3.1. During a grab sample collection, condition the sampling bottle by filling and discarding the sample three times before retaining the fourth sample. 
SECTION: Sample collection and handling for the analysis of primary water quality metrics
4. Collect automated samples as a single discrete sample or composite sample over time. 
SECTION: Sample collection and handling for the analysis of primary water quality metrics
5. Collect runoff and drain tile samples frequently using automated composite sampling through
discharge-weighted methods such as equal discharge increment procedures (APHA, 2005).
SECTION: Sample collection and handling for the analysis of primary water quality metrics
6. Sample the leaching or percolating water using soil water solution sampling lysimeters consisting of porous devices that can extract soil water under a vacuum or access wells to permit sampling beneath a water table. Care must be taken in the selection of lysimeter type with respect to the analytes that are to be measured, as well as in the installation of lysimeters (see reviews by Weihermüller et al. 2007 and Singh et al. 2018).

 

SECTION: Sample collection and handling for the analysis of primary water quality metrics
7. Return samples to the laboratory on ice and filter them on collection day if possible. 

SECTION: Sample collection and handling for the analysis of primary water quality metrics
7.1. Store the subsamples of filtered and unfiltered samples in separate bottles because preservation techniques and storage times differ between analyses. 
SECTION: Quality assurance and quality control: General procedures
8. For the analytical methods described above, follow standard quality assurance (QA) and quality control (QC) procedures (APHA, 2005). 

 

SECTION: Quality assurance and quality control: General procedures
9. Perform instrument maintenance according to the manufacturer’s instructions and maintain a log of the instrument performance.
SECTION: Quality assurance and quality control: General procedures
10. Determine the method detection limit (MDL; the analyte concentration that produces a signal with a 99% probability of being different from the blank) within site-specific water matrices.
SECTION: Quality assurance and quality control: General procedures
11. Field Blanks: Place deionized water in a sampling container in the laboratory, transport it to the field, and treat it as a sample, including exposure to sampling conditions, sample handling (preservation, filtration, and storage), and sample analysis.
SECTION: Quality assurance and quality control: General procedures
12. Laboratory Blanks: To assess contamination from the laboratory environment, treat deionized water as a sample throughout the analytical process. Values should not exceed the MDL.
SECTION: Quality assurance and quality control: General procedures
13. Prepare a standard calibration curve with four to six analyte concentrations. Check for instrument accuracy and precision. 
SECTION: Quality assurance and quality control: General procedures
13.1. Conduct a new calibration at the beginning of each instrument run. Reportable results fall within the range of the standards used in the calibration.
SECTION: Quality assurance and quality control: General procedures
14. Run a certified standard (also termed: instrument performance check standard, calibration check standard, external standard, or standard reference material) with each analytical sequence to verify calibration and method accuracy. 

 

SECTION: Quality assurance and quality control: General procedures
15. Laboratory fortified matrix (LFM): To determine whether the sample matrix contributes bias to the analytical results, add a known amount of analyte to a sample. The LFM is analyzed just like a sample.
SECTION: Quality assurance and quality control: General procedures
15.1. Conduct one LFM duplicate per sample set or on 5% of the samples, whichever is more frequent. Acceptance criterion is 90-110% recovery.
SECTION: Quality assurance and quality control: General procedures
16. Analyze the duplicates to assess precision.
SECTION: Quality assurance and quality control: General procedures
16.1.  If duplicates differ by more than 10%, reanalyze the sample.