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PEARL Platform Methodology & Data Standards

This document describes the measurement parameters, calculation methods, data sources, and quality assurance standards used across the PEARL water quality monitoring platform. PIN ingests data produced by laboratories and agencies that follow EPA-approved analytical procedures and Clean Water Act regulatory frameworks.

Version 1.0·Last updated: February 2026·Project PEARL — project-pearl.org

1.Parameter Definitions

PEARL monitors the following water quality parameters. All measurements are produced by source agencies and laboratories following EPA-approved methods calibrated against NIST-traceable standards.

Parameter	Unit	EPA Method	Detection Limit	PEARL Measurement Approach
Dissolved Oxygen (DO)	mg/L	SM 4500-O G (Membrane electrode)	0.1 mg/L	Optical luminescence sensor (RDO), continuous logging at 15-min intervals, auto-compensated for salinity and temperature
pH	SU (Standard Units)	EPA 150.1 / SM 4500-H⁺ B	0.01 SU	Combination glass electrode with Ag/AgCl reference, 2-point calibration (pH 4.0 & 7.0 NIST buffers), temperature-compensated
Turbidity	NTU	EPA 180.1 (Nephelometric)	0.02 NTU	Nephelometric 90° scatter sensor with ISO 7027 infrared source, auto-wiper to prevent biofouling
Total Suspended Solids (TSS)	mg/L	EPA 160.2 / SM 2540 D	1.0 mg/L	Gravimetric analysis for laboratory QC; field estimates via turbidity-TSS regression calibrated per site (R² ≥ 0.85)
Total Nitrogen (TN)	mg/L	EPA 353.2 (NO₃-N) + SM 4500-N C (TKN)	0.05 mg/L	Ion-selective electrode for nitrate (continuous); persulfate digestion for TKN (grab samples, lab analysis)
Total Phosphorus (TP)	mg/L	EPA 365.1 (Ascorbic acid colorimetric)	0.01 mg/L	Colorimetric in-situ analyzer with persulfate digestion; cross-validated with laboratory ICP-OES quarterly
E. coli	CFU/100 mL or MPN/100 mL	EPA 1603 (mTEC) / SM 9223 B (Colilert)	1 CFU/100 mL	Automated grab sampling with Colilert-18 IDEXX Quanti-Tray; results within 18-24 hrs; geometric mean over 30-day window
Salinity	PSU (Practical Salinity Units)	SM 2520 B (Conductivity ratio)	0.01 PSU	Calculated from conductivity-temperature-depth (CTD) sensor per UNESCO PSS-78 algorithm
Conductivity	µS/cm	EPA 120.1 / SM 2510 B	1 µS/cm	Four-electrode conductivity cell, temperature-compensated to 25°C reference; continuous logging
Temperature	°C	SM 2550 B (Thermometric)	0.01 °C	Platinum RTD sensor (PT-1000), NIST-traceable ±0.1°C accuracy, integrated into multi-parameter sonde

2.Calculation Methods

2.1 Removal Efficiency

Removal efficiency quantifies the percentage reduction of a pollutant concentration between an influent (upstream / pre-treatment) and effluent (downstream / post-treatment) measurement point. For parameters where higher values indicate better quality (e.g., DO), the formula is inverted to reflect improvement.

Standard Removal (TSS, TN, TP, Turbidity, E. coli)

Removal % = ((C_influent − C_effluent) / C_influent) × 100

Where C = concentration in the parameter's native unit. Negative values indicate degradation rather than improvement.

Improvement (DO — higher is better)

Improvement % = ((C_effluent − C_influent) / max(C_influent, 0.1)) × 100

DO efficiency uses the inverse relationship — an increase in DO represents treatment effectiveness. A floor of 0.1 mg/L prevents division-by-zero.

2.2 Health Grades (A–F)

PEARL assigns a composite health grade to each waterbody based on a weighted scoring model across all available parameters. The score is mapped to a letter grade using the following scale:

Score Range	Grade	Interpretation
90–100	A	Excellent — Fully supporting all designated uses; meets or exceeds all criteria
80–89	B	Good — Minor exceedances in 1–2 parameters; generally supporting uses
70–79	C	Fair — Moderate impairment; 2–3 parameters exceed criteria intermittently
60–69	D	Poor — Significant impairment; multiple parameters consistently exceed criteria
0–59	F	Failing — Severe impairment; does not support designated uses; likely Category 5

Composite Score

Score = Σ(w_i × S_i) / Σ(w_i), where i ∈ {DO, pH, Turbidity, TSS, TN, TP, E.coli}

Each parameter score S_i is 0–100 based on distance from the EPA criterion threshold. Weights w_i are: DO=0.20, Nutrients(TN+TP)=0.25, TSS=0.15, Turbidity=0.10, pH=0.10, E. coli=0.20. Parameters with no data are excluded and weights are renormalized.

2.3 TMDL Credit Computation

Total Maximum Daily Load (TMDL) credits represent the pollutant load reduction achieved by a treatment system or BMP, expressed as mass per unit time (typically lbs/day or kg/yr).

Load Reduction (mass/time)

Credit = Q × (C_in − C_out) × CF

Q = flow rate (GPD or L/day); C_in/C_out = influent/effluent concentration (mg/L); CF = unit conversion factor (8.34 × 10⁻⁶ for GPD·mg/L → lbs/day, or 10⁻⁶ for L/day·mg/L → kg/day).

Annualized TMDL Credit

Annual Credit (lbs/yr) = Daily Credit (lbs/day) × Operating Days × Performance Factor

Performance Factor (0.0–1.0) accounts for downtime, maintenance, and seasonal variation. PEARL default = 0.85 for continuous-monitoring systems.

3.Data Sources

PEARL integrates data from multiple federal and state sources. All external data is fetched via official APIs, cached for performance, and timestamped for provenance tracking.

Source	Agency	Update Frequency	Data Type	Coverage
ATTAINS	EPA	Biennial (assessment cycle); cache refreshed daily	Waterbody impairment assessments, 303(d) listings, Category 1-5 classifications, impairment causes	All 50 states + territories; ~2.2M assessment units
Water Quality Portal (WQP)	EPA / USGS / USDA	Continuous; new results within 24–48 hrs	Discrete water quality sample results (physical, chemical, biological)	400M+ results from 900+ organizations nationwide
ECHO	EPA	Monthly permit data; weekly DMR updates	NPDES permit compliance, discharge monitoring reports (DMRs), enforcement actions, inspection history	All active NPDES permits nationwide (~300K facilities)
NPDES	EPA (via ECHO/ICIS)	Monthly	Permit limits, effluent guidelines, compliance schedules, MS4 permit status	All permitted point-source dischargers
NHD (National Hydrography Dataset)	USGS	Annual refresh; HR version quarterly	Stream/waterbody geometry, reach codes, watershed boundaries (HUC-8/10/12), flow direction	Nationwide 1:24,000 scale (NHDPlus HR)
WATERS (ATTAINS GeoServices)	EPA	Synced with ATTAINS cycle	Geospatial impairment mapping, assessment unit boundaries, TMDL linkage	All states with ATTAINS submissions
WDFN	USGS	Real-time (15-min); daily values updated nightly	Streamflow, water level, continuous water quality (DO, pH, turbidity, temperature, conductivity)	~13,000 active real-time stations nationwide
EJScreen	EPA	Annual (demographic); biennial (environmental)	Environmental justice indices, demographic indicators, pollution burden scores per census block group	Nationwide at census block group level
NOAA CO-OPS	NOAA	Real-time (6-min tidal); hourly meteorological	Tidal water levels, water temperature, salinity, meteorological conditions at coastal stations	~210 active stations along US coastline and Great Lakes

4.QA/QC Standards

4.1 Quality Assurance Project Plan (QAPP) Framework

All PEARL monitoring deployments operate under a site-specific QAPP conforming to EPA Requirements for Quality Assurance Project Plans (QA/R-5, EPA/240/B-01/003). The QAPP defines data quality objectives (DQOs), sampling design, analytical methods, and assessment/oversight procedures. Key elements include:

Data Quality Objectives (DQOs): Precision ≤ 20% RPD for field duplicates; accuracy within ±10% of certified reference materials; completeness ≥ 90% for continuous sensors.
Measurement Performance Criteria: Method detection limits (MDLs) verified quarterly per 40 CFR Part 136, Appendix B.
Assessment & Oversight: Internal audits quarterly; external performance evaluation (PE) samples annually through EPA DMRQA program.

4.2 Calibration Requirements

Sensor / Instrument	Calibration Frequency	Standard	Acceptance Criteria
DO (Optical)	Pre-deployment + biweekly	100% air-saturated water + zero-DO (Na₂SO₃)	±0.2 mg/L or ±2% of reading
pH	Pre-deployment + weekly	3-point: pH 4.01, 7.00, 10.01 NIST buffers	Slope 92–102%; offset ±0.3 SU
Turbidity	Pre-deployment + monthly	Formazin: 0, 20, 100, 800 NTU	±5% of known value or ±0.5 NTU below 40 NTU
Conductivity	Pre-deployment + monthly	KCl standards: 100, 1000, 10000 µS/cm	±5% of known value
Temperature	Annual verification	NIST-traceable thermometer	±0.2°C
Nutrient Analyzer (TN/TP)	Each analytical batch	Certified reference materials (ERA/NIST)	Recovery 90–110%; RPD ≤ 15%

4.3 Data Validation Rules

All incoming data — both from PEARL sensors and external APIs — passes through a multi-tier validation pipeline:

Tier	Check	Action on Failure
Tier 1 — Range	Value within physically plausible bounds (e.g., DO 0–20 mg/L, pH 2–14, Temp -2–45°C)	Flagged as suspect; excluded from grade calculations; retained in raw archive
Tier 2 — Rate-of-Change	Parameter change between consecutive readings ≤ 3× historical σ for that site	Flagged; manual review required; interpolated if isolated spike
Tier 3 — Sensor Diagnostics	Battery voltage, signal strength, wiper cycle confirmation, fouling index	Data quarantined; maintenance alert generated
Tier 4 — Cross-Parameter	Logical consistency (e.g., conductivity vs salinity relationship; DO vs temperature saturation)	Lower-confidence flag; both parameters reviewed
Tier 5 — Duplicate/Replicate	Field duplicate RPD ≤ 20%; lab replicate RPD ≤ 15%	Results averaged if within tolerance; flagged if exceeded

4.4 Outlier Detection

PEARL uses a combination of statistical and domain-specific methods to detect outliers:

Modified Z-Score (Iglewicz & Hoaglin): Median-based; flags values with |M_i| > 3.5 where M_i = 0.6745(x_i − median)/MAD.
Grubbs' Test: Applied to grab-sample batches (n ≥ 7) at α = 0.05 significance level.
Seasonal Decomposition: STL decomposition removes seasonal + trend components; residuals beyond ±3σ flagged.
Domain Rules: Supersaturation events (DO > 120% saturation) retained but annotated; sudden salinity drops after rain events are expected and not flagged.

5.Units & Conversions

Standard units and conversion factors used throughout the PEARL platform.

Quantity	PEARL Standard Unit	Conversion	Notes
Concentration (mass)	mg/L	1 mg/L = 1 ppm = 1,000 µg/L	Used for DO, TN, TP, TSS
Concentration (bacteria)	CFU/100 mL	1 CFU/100 mL ≈ 1 MPN/100 mL	EPA considers CFU and MPN interchangeable for regulatory purposes
Turbidity	NTU	1 NTU ≈ 1 FNU (ISO 7027)	NTU (EPA 180.1) and FNU (ISO 7027) are numerically similar but methodologically distinct
Conductivity	µS/cm	1 mS/cm = 1,000 µS/cm	Always reported at 25°C reference temperature
Salinity	PSU	1 PSU ≈ 1 ppt ≈ 1 g/kg	PSU is dimensionless per PSS-78; ppt is approximate equivalent
Temperature	°C	°F = (°C × 9/5) + 32	All PEARL data stored in Celsius
Flow rate	GPD (gallons/day)	1 GPD = 3.785 L/day; 1 MGD = 10⁶ GPD	Used for TMDL load calculations and PEARL unit capacity
Load	lbs/day	1 lb/day = 0.4536 kg/day	TMDL credits and pollutant load reductions
Area	acres	1 acre = 0.4047 hectares = 43,560 ft²	Watershed and drainage area
Volume	gallons	1 gallon = 3.785 liters	Treatment capacity and biofiltration volumes
pH	SU (dimensionless)	pH = −log₁₀[H⁺]	Logarithmic scale: each unit = 10× change in acidity

6.Glossary

Key terms used across the PEARL platform, defined in plain language.

ATTAINS

Assessment, Total Maximum Daily Load (TMDL) Tracking and Implementation System. EPA's national database for tracking waterbody health assessments and impairment listings under Clean Water Act §303(d).

BMP (Best Management Practice)

A structural or non-structural control measure — such as rain gardens, bioswales, retention ponds, or street sweeping — designed to reduce pollutant loads in stormwater runoff before it reaches surface waters.

Category 5 (Cat 5)

The most impaired classification under CWA §303(d). A waterbody in Category 5 is impaired for one or more designated uses, and no TMDL has been established yet. These waterbodies are the highest priority for restoration.

CWA (Clean Water Act)

Federal law (33 U.S.C. §1251 et seq.) establishing the regulatory framework for discharging pollutants into US waters and setting surface water quality standards.

Designated Use

The purpose a waterbody is intended to serve — e.g., aquatic life support, recreation (swimming), drinking water supply, shellfish harvesting, or agricultural irrigation.

DMR (Discharge Monitoring Report)

A periodic report submitted by NPDES-permitted facilities documenting their actual effluent discharge concentrations and volumes, filed through EPA's NetDMR system.

DO (Dissolved Oxygen)

The amount of oxygen gas dissolved in water. Aquatic life requires minimum DO levels — typically ≥ 5.0 mg/L for most freshwater fish species.

DQO (Data Quality Objective)

Qualitative and quantitative statements that define the type, quality, and quantity of data needed to support a specific decision or action, per EPA Guidance (QA/G-4).

EJ (Environmental Justice)

The fair treatment and meaningful involvement of all people — regardless of race, color, national origin, or income — in environmental decision-making. EPA's EJScreen tool quantifies EJ vulnerability.

GPD (Gallons Per Day)

Standard unit for flow rate in US water treatment contexts. PEARL units are rated in GPD to indicate treatment throughput capacity.

Impairment

A waterbody is "impaired" when it fails to meet water quality criteria for one or more of its designated uses. Impairments are classified by severity (Categories 1–5) and by cause (e.g., nutrients, pathogens, sediment).

MDL (Method Detection Limit)

The minimum concentration of a substance that can be measured with 99% confidence that the true value is greater than zero, per 40 CFR Part 136 Appendix B.

MS4 (Municipal Separate Storm Sewer System)

A publicly-owned stormwater conveyance system (storm drains, pipes, ditches) that discharges into surface waters without treatment. MS4 operators require NPDES permits and must implement stormwater management programs.

NPDES (National Pollutant Discharge Elimination System)

The EPA permit program under CWA §402 that regulates point-source discharges of pollutants into US waters. All MS4s, industrial facilities, and wastewater treatment plants require NPDES permits.

NTU (Nephelometric Turbidity Unit)

The standard unit for measuring turbidity — the cloudiness of water caused by suspended particles. Higher NTU values indicate more turbid (cloudier) water.

QAPP (Quality Assurance Project Plan)

A formal document describing the quality assurance and quality control activities for an environmental data collection project, per EPA Requirements QA/R-5.

Removal Efficiency

The percentage reduction of a pollutant achieved by a treatment system, calculated from influent and effluent concentrations. A 90% TSS removal means the system captures 90% of suspended solids.

TMDL (Total Maximum Daily Load)

The maximum amount of a pollutant that a waterbody can receive and still meet water quality standards. TMDLs are established for impaired waters under CWA §303(d) and allocate pollutant loads among sources.

TSS (Total Suspended Solids)

The total mass of solid particles (organic and inorganic) suspended in a water sample, measured by filtration and gravimetric analysis. A primary indicator of sediment pollution.

WLA (Wasteload Allocation)

The portion of a TMDL assigned to existing and future point sources (e.g., NPDES-permitted dischargers). The remaining allocation goes to nonpoint sources (LA) and a margin of safety (MOS).

WQS (Water Quality Standards)

State-adopted standards defining designated uses, numeric criteria (e.g., DO ≥ 5.0 mg/L), narrative criteria, and antidegradation policies for each waterbody, per CWA §303(c).

PEARL Platform Methodology & Data Standards · Version 1.0 · February 2026

This document is provided for informational purposes. PEARL assessments are derived from publicly available data and do not constitute official regulatory determinations. Always verify with primary agency sources for compliance or permitting decisions.

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