What Cannabis in Wastewater Can Reveal About Local Use

Every time cannabis is consumed, the evidence does not disappear when its immediate effects wear off. The human body processes cannabinoids and excretes some of the resulting compounds. These residues enter municipal sewer systems, where researchers can detect them in wastewater and use them to examine cannabis activity across an entire community.

This approach, known as wastewater-based epidemiology, offers something that surveys, retail data, and law enforcement statistics cannot provide on their own. It captures a pooled and largely anonymous chemical signal generated by the population connected to a wastewater system.

However, interpreting that signal is far more difficult than simply detecting a cannabinoid.

A new systematic review published¹ in Environmental Toxicology and Pharmacology demonstrates how strongly seasons can influence concentrations of pharmaceuticals and illicit drugs in wastewater and surface water. After examining 247 studies covering 68 countries, the researchers found no universal season in which concentrations were consistently highest.

For cannabis monitoring, this means a higher concentration of cannabinoid residues cannot automatically be interpreted as proof that more cannabis was consumed. Weather, tourism, wastewater volume, chemical degradation, and treatment performance can all change what reaches the sampling instrument.

How Cannabis Consumption Leaves a Chemical Signature

After a person consumes cannabis, cannabinoids such as THC are absorbed, metabolized, and eventually eliminated from the body. Wastewater researchers generally look for the original compounds or their metabolites, which are chemicals produced as the body breaks those compounds down.

Samples collected as wastewater enters a treatment facility are described as influent. Samples taken after treatment are called effluent. Researchers can also test rivers, lakes, and other surface waters receiving treated wastewater.

Each matrix answers a somewhat different question. Influent is most closely connected to what a community has recently consumed or discarded. Effluent indicates what remains after treatment. Surface water shows how residues behave after entering a larger and more complex environment.

Canada already applies this broader concept through its National Wastewater Drug Surveillance initiative, which analyzes municipal wastewater for drugs, metabolites, and precursor chemicals. Similar programs operate elsewhere, including the European Union Drugs Agency wastewater network.

The appeal is straightforward. Wastewater data do not depend on people accurately remembering or admitting what they consumed. They can also reveal changes faster than annual surveys. At the same time, wastewater measurements describe a collective population rather than identifying individual consumers.

Cannabinoid Levels Change With the Seasons

The new review examined research published between 2004 and 2021. It covered pharmaceuticals and several illicit-drug groups across influent wastewater, effluent wastewater, and surface water.

Cannabinoids represented only one part of this much larger dataset, but the results demonstrate why cannabis wastewater measurements require seasonal context.

In European effluent wastewater, cannabinoid concentrations were significantly higher in summer and winter than in spring. The median summer concentration was 26.5 nanograms per litre, while the winter median was 24.0 nanograms per litre. Spring produced a considerably lower median of 11.0 nanograms per litre.

The pattern is notable because summer and winter represent very different environmental conditions. If cannabinoid concentrations were controlled only by cannabis use, one might expect a simpler progression. Instead, two contrasting seasons produced similarly elevated medians.

The study also found that cannabinoid prevalence remained relatively stable during one Australian investigation, even as cocaine concentrations increased over the Christmas and New Year holiday period. This reinforces the point that seasonal events do not affect every substance in the same way.

Why More Cannabinoids Do Not Necessarily Mean More Cannabis Use

Wastewater concentration reflects everything that happens between consumption and sampling. A community may consume the same amount of cannabis in two seasons yet produce different wastewater readings.

Several variables can alter the detected concentration:

• Rainfall can dilute wastewater and receiving rivers.
• Heat and sunlight can accelerate chemical degradation.
• Tourism can temporarily change the population being measured.
• Treatment performance can vary with temperature and water flow.
• Compounds can transform while moving through sewer infrastructure.

Rainfall, wastewater flow, and changing environmental conditions can alter cannabinoid concentrations independently of consumption.

Population movement is particularly important. A treatment facility serving a tourist destination might receive wastewater from far more people during vacation periods. A rising cannabinoid concentration could reflect greater cannabis use, a larger temporary population, lower water flow, or several of these factors at once.

The opposite problem is also possible. Heavy rainfall may dilute cannabinoid residues enough to produce a lower concentration even if consumption has increased. Researchers can partly address this by measuring wastewater flow and estimating the number of people contributing to the system, but those estimates introduce additional uncertainty.

This is why the paper warns against treating influent concentrations as direct measurements of consumption. The researchers recommend combining concentration data with flow-normalized loads, population estimates, weather information, hydrological measurements, and standardized sampling over time.

Wastewater Could Reveal Changes After Cannabis Legalization

Despite these limitations, wastewater monitoring has significant value for regulated cannabis markets. Governments generally assess cannabis consumption using household surveys, legal sales, medical registrations, and enforcement data. Each source captures only part of the market.

Legal sales exclude products obtained from unlicensed sellers or grown at home. Surveys can be affected by incomplete recall and reluctance to disclose drug use. Arrest and seizure data reflect enforcement activity as much as underlying consumption.

Wastewater provides an independent measurement that can help test whether these indicators are moving in the same direction. Researchers have already used sewage data to study cannabis consumption before and after legal retail sales began in Washington State. The results showed that legal sales and wastewater-derived estimates did not increase at the same rate, illustrating both the technique’s potential and its remaining uncertainty.

For Canada, that creates an opportunity to compare cannabinoid residues with legal retail performance. The regulated industry has become a substantial economic sector, with cannabis contributing billions of dollars to economic activity since legalization, as examined in MyCannabis coverage of Canada’s cannabis economy.

If legal sales rise while population-adjusted wastewater loads remain stable, the change could suggest that consumers are moving from illicit suppliers to regulated stores rather than consuming substantially more cannabis. If both measurements rise, overall use may be increasing. Neither conclusion should be made from one measurement alone, but the combination is considerably more informative than either dataset in isolation.

Cannabis Residues Are Also an Environmental Question

Wastewater analysis is not solely a method for estimating community behavior. It can also show whether cannabinoid residues survive treatment and enter the environment.

The detection of cannabinoids in effluent demonstrates that treatment does not necessarily remove every trace. That does not automatically establish ecological harm. Environmental risk depends on concentration, persistence, exposure duration, toxicity, and whether compounds accumulate or transform after discharge.

The review found that seasonal variation was more commonly detected in wastewater than in surface water, although surface water was also much less extensively studied. When contaminants reach a river, rainfall, flow, sunlight, temperature, sediment, and biological activity create additional variables that can weaken or reshape the original wastewater signal.

This distinction matters for regulators. A cannabinoid may be useful as a marker of community consumption at the treatment plant inlet but unreliable for that purpose farther downstream. Conversely, its continued presence after treatment may be more useful for evaluating treatment performance and environmental exposure.

Better analytical methods will improve both applications. MyCannabis has previously examined how automated cannabis testing can reduce sample and solvent requirements while preserving laboratory performance. Comparable improvements in automation, sensitivity, and standardization could make routine cannabinoid monitoring more practical for wastewater facilities.

The Missing Global Cannabis Wastewater Map

One of the paper’s most important findings is how uneven the available evidence remains. Europe accounted for 43.48 percent of the included studies, followed by Asia at 30.75 percent and North America at 13.98 percent. Africa, South America, and Oceania collectively represented less than 12 percent.

Even within continents, data were concentrated in a limited number of countries. China contributed more than 6,000 data points, while some countries contributed only one. The apparent global picture is therefore shaped heavily by the places with established laboratories, monitoring programs, and publication capacity.

The four-season framework also fits temperate regions better than tropical and equatorial climates. In many locations, wet and dry seasons influence wastewater and rivers more meaningfully than spring, summer, autumn, and winter. Applying a northern seasonal model everywhere could conceal the environmental conditions actually controlling cannabinoid concentrations.

A more useful global system would organize monitoring around locally relevant variables such as precipitation, river flow, temperature, tourism, and wastewater-treatment capacity. Cannabis legalization status should also be considered because legal access, product availability, reporting behavior, and illicit-market activity can all affect how wastewater results are interpreted.

Wastewater Is Evidence, Not a Verdict

The promise of cannabis wastewater monitoring lies in its ability to observe populations without relying entirely on surveys, purchases, or enforcement. It could help governments evaluate legalization, identify regional changes, study treatment performance, and monitor environmental discharge.

Its weakness is that a wastewater sample compresses many human and environmental processes into one number. Consumption is part of that number, but so are rainfall, population movement, sewer conditions, chemical stability, and treatment efficiency.

The new review’s most useful conclusion is therefore not that cannabis wastewater levels peak during one particular season. It is that there is no universal peak. Meaningful monitoring must be regional, repeated across seasons, and interpreted alongside environmental and population data.

Wastewater can reveal a great deal about cannabis use and its environmental footprint. It simply cannot tell the full story by itself.

References:

^{1. Coelho, F. D. A., Morgan, R., & Campos, L. C. (2026). Pharmaceuticals and illicit drugs in environmental waters: A systematic review and quantitative analysis of global trends 2004–2021. Part 2. Environmental Toxicology and Pharmacology, 105164. https://doi.org/10.1016/j.etap.2026.105164}