Studying for 2026–27? Use the current marine/estuary study guide. For the season-pure historical material, open the 2025–26 freshwater archive guide. Other resources on this legacy page may span seasons.
Study Guide
Expert-designed curriculum in 12 modules, tagged by priority and difficulty.
Open 2025–26 Guide →
Flashcards
Quick-review card deck with spaced repetition for efficient memorization.
Study Flashcards →
Past Exams
Practice with real MIT invitational questions. Official answer keys + AI explanations.
Browse Past Exams →
📺 Study Videos
View all videos →
🔢 Key Numbers — Quick Reference
Essential thresholds and ranges for exam day. Bookmark this.
💧 Dissolved Oxygen
Excellent> 8 mg/L
Good6–8 mg/L
Stressed4–6 mg/L
Hypoxic< 2 mg/L
Saturation (20 °C)9.1 mg/L
⚗️ pH Range
Optimal aquatic6.5–8.5
EPA drinking water6.5–8.5
Acid rain< 5.6
Neutral7.0
📊 WQI Scores
Excellent91–100
Good71–90
Medium51–70
Bad26–50
Very Bad0–25
🦟 HBI (Hilsenhoff)
Excellent0.00–3.50
Very Good3.51–4.50
Good4.51–5.50
Fair5.51–6.50
Poor / Very Poor> 6.50
🚰 Key MCL Values
Nitrate (NO₃⁻)10 mg/L
Phosphate (PO₄³⁻)0.1 mg/L
Lead (Pb)0.015 mg/L
Arsenic (As)0.010 mg/L
Total Coliform0 (absent)
🔬 Turbidity
Drinking water MCL1 NTU
Clear stream< 10 NTU
Slightly turbid10–25 NTU
Turbid25–100 NTU
Very turbid> 100 NTU
🔗 Reference Resources
📖
Competition strategy page covering ecosystem effect chains, turbidity impacts, eutrophication chains, and WQI parameter …
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Detailed build log for a Science Olympiad digital salinometer (2012). Covers opamp circuit, Wien bridge oscillator, Pelt…
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Step-by-step salinometer build guide covering conductivity measurement, electrode design, and temperature correction (~2…
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USGS fact sheet covering mercury sources, methylation in aquatic sediments, bioaccumulation in fish, and biomagnificatio…
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EPA's Rapid Bioassessment Protocols for Use in Streams and Wadeable Rivers — the standard reference for macroinvertebrat…
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Official 2026 general handout covering all four parts of the event. Binary PDF.
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Official soinc.org Part 3 handout covering freshwater ecosystem analysis, zone structure, organism identification, and b…
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Official guide for building and using the salinometer/hydrometer device. Describes straw/clay basic hydrometer design an…
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Official 2019 sample test for freshwater Water Quality. Binary PDF.
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Official practice activities including Q-value curves, WQI calculations, and BOD practice problems. Key source for WQI w…
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Detailed explanation of EPT/C ratio, HBI calculation, and other macroinvertebrate biomonitoring metrics used in field as…
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NRCS Watershed Science Institute technical note covering IBI metrics, scoring, and relationship to impervious surface ar…
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Brochure explaining the fish-based Index of Biotic Integrity, its metrics, and application. Cites Karr et al. developmen…
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EPA's comprehensive guide for citizen monitoring of stream water quality — covers physical/chemical/biological parameter…
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EPA Office of Science and Technology primer on water quality standards — designated uses, criteria, antidegradation, and…
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EPA collection of technology fact sheets for wastewater treatment processes — covers activated sludge, trickling filters…
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EPA factsheet covering DO measurement, standards, ecological effects, and regulatory context for dissolved oxygen as a w…
🛠️
Interactive EPA map tool showing water quality status (impaired/attaining) for streams, lakes, and coasts nationwide — d…
🛠️
USGS database for real-time and historical water quality data from thousands of monitoring stations nationwide — searcha…
🛠️
Online calculator for dissolved oxygen saturation based on temperature, salinity, and altitude using standard equations.
🔗
Concise reference for the HBI formula (∑(nᵢ × tᵢ) / N), tolerance value tables, and water quality rating interpretation …
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Step-by-step guide to calculating EPT index and %EPT by dividing EPT totals by all macroinvertebrates, with data interpr…
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Clear explanation of kick-net methodology with instructions on net placement, substrate disturbance, and qualitative vs.…
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Official Ohio EPA page explaining the QHEI methodology for evaluating stream physical habitat quality, with links to dat…
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EPA overview of CWA structure, covering Sections 301 (discharge prohibition), 402 (NPDES permit program), 404 (dredge/fi…
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EPA's official TMDL overview explaining loading capacity, waste load allocation, load allocation, and the process for re…
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The official EPA handbook covering all three components of water quality standards (designated uses, water quality crite…
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USGS Water Science School article explaining nitrogen forms in water (ammonia, nitrite, nitrate), sources, health effect…
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EPA's HABs overview covering algae types that produce toxic blooms, cyanotoxin categories (hepatotoxins, neurotoxins), h…
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NALMS reference page with the three Carlson TSI equations (Secchi depth, total phosphorus, chlorophyll-a), the 0–100 ind…
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Clear applied explanation of carbonate buffering and alkalinity, covering how bicarbonate/carbonate species maintain pH …
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Well-organized academic reference distinguishing temporary vs. permanent hardness, the chemistry of each type, measureme…
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EPA overview of CWA Section 404, which regulates discharge of dredged or fill material into wetlands and waters of the U…
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North American Invasive Species Management Association overview covering zebra mussels, Eurasian watermilfoil, and Asian…
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Clear state-agency explanation of how DBPs (THMs and HAAs) form when chlorine reacts with natural organic matter, regula…
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EPA's comprehensive BMP menu covering detention/retention basins, bioswales, permeable pavement, green roofs, and other …
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Practical reference explaining TDS definition, the relationship to conductivity (~100× cation equivalents), measurement …
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Thorough reference covering the BOD₅ test procedure (dilution method, seed preparation, DO measurements at day 0 and day…
🌐
Official Science Olympiad Water Quality event page; confirmed 2026 freshwater focus.
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Repository of past invitational/regional tests. Water Quality tests available. Note: Anubis bot protection may block dir…
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Community-maintained event wiki with study notes, resources, and strategies.
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Community thread for organism identification practice and discussion.
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Community strategies, device build ideas, and test-taking tips for all SO events.
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Curated links and study materials for Water Quality event.
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University of New Hampshire Center for Freshwater Biology resource explaining HBI formula, tolerance values, and worked …
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Table of family-level tolerance values for HBI calculation. Access may be intermittent; confirmed via search snippet.
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Official EPA page listing all primary MCLs and MCLGs for drinking water contaminants. Key source for nitrate, lead, merc…
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EPA page listing non-enforceable secondary standards for aesthetic properties: TDS (500 mg/L), iron, manganese, pH, colo…
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Overview of nutrient (nitrate) regulations and algal bloom dynamics in freshwater.
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Hackaday coverage of Kramnik's salinometer with summary of circuit design and principles.
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Guide covering bioaccumulation of heavy metals in water, health effects, and key metals (lead, mercury, arsenic, cadmium…
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Overview of heavy metal bioaccumulation processes and persistence in aquatic systems.
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Peer-reviewed paper on heavy metal bioaccumulation and biomagnification in fish across different aquatic habitats.
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Academic publication reviewing WQI methodologies including NSF WQI score interpretation (Excellent/Good/Medium/Bad/Very …
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EPA landing page for volunteer water quality monitoring programs — links to both stream and lake monitoring manuals, dat…
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USGS hub for water quality education topics including pH, DO, turbidity, nutrients, temperature, bacteria, and conductiv…
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North American Lake Management Society's citizen science Secchi depth monitoring program — explains technique, data inte…
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EPA technical page with criteria for un-ionized ammonia toxicity at varying pH and temperature — includes the key equati…
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USGS interactive overview of freshwater distribution, including groundwater vs. surface water percentages and the global…
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National Water Quality Assessment (NAWQA) pesticide national synthesis — data, trends, and research on pesticide occurre…
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Interactive map and database of nonindigenous aquatic species (invasive species) across US watersheds, updated continuou…
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Comprehensive EPA resource on nutrient pollution sources, effects, and solutions — covers nitrogen, phosphorus, eutrophi…
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EPA's PFAS page covering the 2024 maximum contaminant levels (4 ppt for PFOA/PFOS), health effects, and regulatory histo…
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EPA 2013 ammonia criteria document detailing acute and chronic thresholds for un-ionized ammonia, pH and temperature cor…
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EPA resource on green infrastructure approaches to stormwater management — rain gardens, bioswales, permeable pavement, …
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EPA overview of Total Maximum Daily Loads — the regulatory framework for restoring impaired water bodies under Clean Wat…
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National survey data on condition of rivers, streams, lakes, wetlands, and coastal waters — includes WQI components and …
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Key 2020 paper on freshwater biodiversity threats and conservation strategies. Useful context for understanding watershe…
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Landmark PNAS paper arguing eutrophication cannot be controlled by reducing nitrogen alone — phosphorus is the key limit…
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Original 1977 paper introducing Carlson's Trophic State Index (TSI) using Secchi depth, chlorophyll-a, and total phospho…
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Foundational paper introducing the River Continuum Concept — how stream communities change from headwaters to mouth base…
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Comprehensive 2023 review of water quality index methods, including NSF WQI variants, parameter weighting schemes, and g…
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NOAA educational resource on ocean acidification — CO₂ chemistry, pH changes, effects on calcifying organisms, and monit…
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Near real-time satellite monitoring of coral reef thermal stress and bleaching alerts worldwide.
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NOAA's harmful algal bloom monitoring, forecasting, and research program — covers marine and freshwater HABs, toxins, an…
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Louisiana Universities Marine Consortium resource on the Gulf of Mexico dead zone — size data, causes (nutrient loading …
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Overview of the Streeter-Phelps dissolved oxygen sag curve equation — the foundational model for BOD and DO dynamics in …
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University of New Hampshire interactive stream key for identifying macroinvertebrate orders used in bioassessment — incl…
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IISD Experimental Lakes Area explanation of lake thermal stratification, thermocline formation, and fall/spring turnover…
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NOAA interactive tutorial covering estuary types, salinity gradients, ecosystem services, and the importance of estuarie…
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Smithsonian Institution's comprehensive ocean science portal — covers marine habitats, ocean chemistry, coral reefs, and…
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Open-source environmental engineering lab textbook chapter on acid precipitation — carbonate buffering, alkalinity titra…
🦟
Macroinvertebrate Visual Quiz
Identify macroinvertebrates from photos — builds Part III skills fast.
🎯 Start Visual Quiz