Water Quality Monitoring

PRLT has participated in the Maine Department of Environmental Protection (DEP) Volunteer River Monitoring Program (VRMP) for over a decade. Each spring, a group of dedicated PRLT volunteers are trained by Maine DEP and PRLT staff on how to use handheld meters, fill out data sheets, and complete quality assurance duplicate sampling. These volunteer Water Stewards then conduct monitoring every two weeks from May through September at sites throughout the Presumpscot River watershed region. At each of the sites, the volunteers take measurements of dissolved oxygen (DO), conductivity, and temperature using a YSI meter. For measuring levels of E. coli bacteria, “grab samples” are collected and transported to the PRLT office for analysis using the IDEXX Quanti-Tray 2000 method.

We collect data from the Presumpscot River, its major tributaries—Pleasant River, Little River, and Piscataqua River—and other small tributaries including Mill Brook, Otter Brook, Black Brook, Colley Wright Brook, Inkhorn Brook, Ditch Brook, Douglas Brook, Tannery Brook, and Thayer Brook. Though not part of the Presumpscot River watershed, PRLT also monitors the Stroudwater River.

In 2026, Casco Bay Estuary Partnership lent PRLT twelve Onset HOBO MX801 Series Water Data Loggers. These continuous loggers help us monitor dissolved oxygen and conductivity levels more closely at sites along the main stem of the Presumpscot River (read more about the continuous meters here). We will analyze the data for quality assurance, trends, and significance, and share the results on our website in winter 2026/2027.

What our monitoring measures

E. coli Bacteria – Grab samples are taken at each sampling site and tested for Escherichia coli (E. coli). E. coli is a bacteria primarily associated with the gut of humans and other warm-blooded animals. Although it is not typically a native organism of aquatic ecosystems, it can enter and persist in water and sediment through fecal contamination and may be carried by aquatic organisms, including fish and invertebrates. Elevated E. coli concentrations can indicate fecal pollution potentially from sewage, agricultural runoff, urban pet waste, or wildlife, which affects the safety of recreation and fishing for people using those water bodies. It can also contribute to increased turbidity (cloudiness) of water and reduced dissolved oxygen levels for aquatic life.

Dissolved Oxygen – Dissolved oxygen (DO) readings are taken at each sampling site using a meter. DO reflects the habitat quality of water for wildlife, measured in the units milligrams per liter (mg/L) and percent saturation [%]. Healthy levels of DO in the water column are crucial for aerobic processes like decomposition and respiration, while aquatic plants and algae produce oxygen through photosynthesis. Excess nutrients, such as nitrogen and phosphorus, can promote algal blooms that may lead to oxygen depletion when algae die and decompose. Colder water generally holds more dissolved oxygen than warmer water under the same conditions. Fast-moving streams and rivers often have higher oxygen levels because turbulence increases the transfer of oxygen from the atmosphere into the water. DO levels are also influenced by many factors, including temperature, nutrient availability, organic matter, water depth, and the time of day. Fast-moving streams and rivers absorb more oxygen from the atmosphere than stagnant waters, ponds, or deep water.

Conductivity – Specific conductance is measured at each sampling site using a meter. Specific conductance indicates the ability of water to conduct an electrical current and provides an indirect measure of the concentration of dissolved ions, such as salts and minerals. Specific conductance is adjusted with a temperature coefficient and is measured in microsiemens per centimeter (µS/cm). Dissolved ions may originate from natural sources, such as the weathering of rocks and minerals and groundwater inputs, as well as human activities, including road salt application, agricultural runoff, and wastewater discharge. Pure water is a poor conductor of electricity because it contains very few dissolved ions, while water with higher concentrations of dissolved ions generally has higher specific conductance. Although it is a useful indicator of water quality, it does not identify the specific dissolved substances present.

Temperature – Water temperature is measured at each sampling site using a meter and recorded in degrees Celsius (°C). Temperature influences many biological processes in aquatic organisms, including respiration, growth, and reproduction. Warmer water generally holds less dissolved oxygen than colder water under the same conditions, potentially stressing fish and other aquatic organisms with reduced oxygen availability and increased oxygen demands. Temperature could also influence the solubility and toxicity of certain metals and other contaminants. In addition, warmer temperatures can favor algal blooms, when excess levels of nitrogen and phosphorus are present. These blooms are often more common during summer or unusually warm periods.

How Averages and Trends are calculated

Our analysis incorporates historical data dating back to 2009. We monitor and display this data based on the Maine Department of Environmental Protection (DEP) standards for river classification.

For bacteria levels, Maine DEP requires all Class AA, A, B, and C rivers to have E. coli levels less than 236 MPN (Most Probable Number) and CFU (Colony Forming Unit) per 100 milliliters in more than 10% of the samples in any 90-day interval.

• For our analysis, any bacteria result exceeding 236 MPN/100 mL is considered a “fail.”

For dissolved oxygen (DO) levels, Maine DEP requires all Class AA, A, and B rivers to have DO levels greater than or equal to 7 parts per million or 75% of saturation, whichever is higher, and Class C rivers no less than 5 parts per million or 60% of saturation, whichever is higher.

• For our analysis, any DO level under 5 mg/L or 60% is considered a “fail,” despite most sites within the upper portion of the Presumpscot River and tributaries falling into the classifications of A or B.

The volunteer sampling season is biweekly, totaling 10 sample days, from May to September (with a very occasional day missed due to equipment failure or other issues). We take the sum of “failures” at each site compared to the total number of samples taken to find the average percentage of samples taken that meet DEP standards.

For the Historical Water Quality Map, we identify trends and averages in dissolved oxygen and bacteria at each site from 2009 to the present year.

• Averages shown on the Historical Map reflect the percentage of total samples that met the Maine DEP Standards for bacteria or dissolved oxygen levels.
• Trends were calculated from raw DO and bacteria data (the actual levels rather than the performance relative to DEP standards). First, all samples taken in a year were averaged. This resulted in yearly average numbers for each site. Linear regression analysis was then performed to determine if there was a significant trend (p = 0.10) for the site and whether that trend was positive or negative. All sites that showed no statistically significant trend are shown in the map as diamonds. Sites with no trend symbology did not have enough samples to calculate a trend.