Blogs Agricultural Runoff Testing: Adding Endocrine and Xenobiotic Activity Screening

Agricultural Runoff Testing: Adding Endocrine and Xenobiotic Activity Screening

Agricultural runoff can contain more than nutrients and sediment. As rainwater, snowmelt, and irrigation return flows move across fields and agricultural facilities, they may collect pesticides, veterinary compounds, natural hormones, manure-associated contaminants, plant-derived chemicals, and transformation products. These substances can enter drainage systems, streams, ponds, and groundwater as complex mixtures.

CYP Inhibition

Chemical residue analysis remains essential for identifying and quantifying known contaminants. However, chemical measurements alone may not reveal whether a prepared sample or sample extract produces a measurable biological response.

Cell-based reporter assays add functional information to agricultural water testing. They can screen samples for activity associated with endocrine and xenobiotic receptors, as well as cellular stress responses. When combined with targeted or non-targeted chemical analysis, these assays can help researchers identify biologically active samples, prioritize additional testing, and monitor changes across locations or seasons.

Why Agricultural Runoff Is Difficult to Characterize

Agricultural runoff is not a single, consistent sample type. Its composition can change based on:

  • Crop and livestock activity
  • Soil and drainage characteristics
  • Pesticide and fertilizer applications
  • Manure-management practices
  • Rainfall and irrigation patterns
  • Time between chemical application and sampling
  • Environmental degradation and transformation processes

Potential constituents include pesticides, formulation ingredients, veterinary pharmaceuticals, natural and synthetic hormones, plant metabolites, phytoestrogens, manure-derived compounds, and sediment-bound chemicals.

The U.S. Environmental Protection Agency identifies sediment, nutrients, pesticides, pathogens, salts, and other pollutants as materials that may be transported from agricultural operations into surrounding waters.

This variability makes runoff difficult to characterize using a short list of predetermined analytes. Compounds not included in a targeted chemical panel may be overlooked, and transformation products may differ from the chemicals originally applied. Multiple compounds acting through the same receptor may also contribute to a measurable response, although matrix effects and interactions among mixture components can influence the result.

What Reporter Assays Add to Chemical Testing

Chemical analysis and cell-based reporter assays answer different but complementary questions.

Chemical analysis can determine:

  • Which targeted substances are present
  • The concentrations of those substances
  • Whether measured concentrations exceed a benchmark or reporting threshold

Reporter assays can help determine:

  • Whether a prepared sample or sample extract activates or inhibits a selected receptor
  • Whether the sample produces a measurable cellular stress response
  • How biological activity compares across locations, time points, or runoff-management conditions
  • Which samples may warrant additional chemical investigation

A reporter assay response does not identify the compound responsible or demonstrate that a particular agricultural practice caused the activity. It indicates that one or more substances in the tested sample produced a response under the selected assay conditions.

When activity is detected, researchers may use targeted chemical analysis, suspect screening, non-targeted analysis, or effect-directed analysis to investigate potential drivers. Bioassays should therefore be used as screening and prioritization tools—not as replacements for chemical identification, ecological studies, or regulatory risk assessment.

This complementary role is consistent with the OECD’s discussion of effect-based monitoring, which describes how biological measurements can supplement chemical monitoring of endocrine-active substances in freshwater.

Selecting Relevant Assay Endpoints

The appropriate assay panel should be selected according to the monitoring objective, expected contaminants, land use, and available information about chemical applications.

Potential endpoints include:

  • Estrogen Receptor (ER): Detects estrogenic or anti-estrogenic activity that may be associated with natural and synthetic hormones, certain pesticides, plant-derived compounds, and other environmental contaminants.
  • Androgen Receptor (AR): Measures androgenic or anti-androgenic activity, including activity from substances that may interfere with normal androgen receptor signaling.
  • Aryl Hydrocarbon Receptor (AhR): Screens for activity associated with structurally diverse xenobiotics, including certain aromatic and dioxin-like compounds.
  • Glucocorticoid Receptor (GR): Detects compounds that activate or inhibit glucocorticoid receptor signaling, including some natural and synthetic steroidal substances.
  • Nrf2: Measures a cellular response associated with oxidative or electrophilic stress. Unlike ER, AR, AhR, and GR, Nrf2 is a transcription factor rather than a receptor and provides a complementary stress-response endpoint.

Together, these assays can provide a broader functional profile of a sample. Results should be interpreted alongside cytotoxicity, matrix-interference controls, sample-processing information, and relevant chemical data.

A Practical Runoff-Screening Workflow

An agricultural runoff study can be organized around five general steps.

1. Define the Monitoring Question

Determine whether the study is intended to:

  • Identify samples with measurable biological activity
  • Compare upstream, field-edge, and downstream locations
  • Track changes following pesticide or manure application
  • Evaluate seasonal trends
  • Compare runoff before and after implementation of management practices

The monitoring question should guide assay selection, sampling frequency, and chemical-analysis plans.

2. Select Sampling Locations and Time Points

One-time sampling provides only a snapshot. Agricultural practices and weather conditions change throughout the year, and runoff activity may change with them.

Potential sampling points include:

  • Before major chemical applications
  • Shortly after pesticide or fertilizer application
  • During the first substantial rainfall after application
  • During periods of high irrigation return flow
  • During manure-application periods
  • Throughout planting and growing seasons
  • After harvest
  • At upstream, field-edge, and downstream locations

Event-based sampling can be especially valuable because the first major runoff event after a dry period or chemical application may differ from routine baseflow conditions.

3. Standardize Sample Preparation

Sample collection, storage, filtration, extraction, concentration, and dilution procedures should be standardized when results will be compared across time points or locations.

The selected preparation method can affect which chemicals are retained and ultimately tested. Sample-preparation decisions should therefore be documented and considered when interpreting results.

4. Screen for Biological Activity

Prepared samples or extracts can be evaluated using selected receptor and stress-response assays. Appropriate controls should be included to assess:

  • Assay performance
  • Cytotoxicity
  • Matrix interference
  • Sample recovery
  • Potential solvent effects
  • Concentration-dependent responses

Testing a dilution or concentration series can help distinguish a reproducible assay response from cytotoxicity or nonspecific interference.

5. Investigate Active Samples

Samples that produce measurable activity can be prioritized for additional chemical investigation. Comparing biological results with chemical measurements may help determine whether known residues account for the observed activity or whether additional analysis is warranted.

Add Functional Screening to Agricultural Water Studies

Agricultural runoff can contain dynamic mixtures that are difficult to characterize through targeted residue analysis alone. Cell-based reporter assays can complement chemical testing by identifying samples with measurable endocrine receptor, xenobiotic receptor, or Nrf2-mediated stress-response activity.

INDIGO Biosciences offers reporter assay kits for laboratories conducting testing internally and assay services for organizations that want to outsource sample screening. Available endpoints include ER, AR, AhR, GR, and Nrf2, with assay selection based on the study objective, sample type, and compounds of concern.

Planning an agricultural water or runoff study? Talk with INDIGO about sample preparation, assay selection, testing format, and study design.