Surface Water and Groundwater Quality

What happens to a receiving water: stream degradation and oxygen dynamics, total maximum daily loads, and biological and chemical contaminants. The NCEES spec gives it 5–8 of 80 questions. The math is mostly mixing plus first-order decay; the definitions carry the points.

What the problems look like

River-reach problems with an outfall and a time of travel. Recurring shapes — mix an effluent into a stream (flow, BOD, temperature, DO), then run the Streeter–Phelps sag to a downstream point or the critical point; BOD exertion from a 5-day value to ultimate, with a temperature correction; reaeration coefficients; TMDL arithmetic — loading capacity, waste load allocations, load allocation, margin of safety; coliform die-off below an overflow; and exposure and toxicity — chronic daily intake, reference dose, sorption (Kd = Koc·foc), and bioconcentration from Kow.

By exam day you should be able to —

• mix an effluent and a stream on flow-weighted concentration and temperature;
• compute the deficit and DO at a downstream point, and the critical deficit;
• convert BOD₅ to ultimate BOD and correct k for temperature;
• build a TMDL — ΣWLA + ΣLA + MOS — and solve for any one term;
• compute chronic daily intake for carcinogen and noncarcinogen cases, and a reference dose from a NOAEL;
• compute a retardation factor from Koc·foc and slow a plume's travel accordingly, and decay a coliform count at the stated temperature.

Where people lose points

• Deficit vs. DO. The deficit is saturation minus measured DO, read at the mixed temperature (and salinity).
• Base e vs. base 10. Rate constants come in both; convert to base e (k = 2.303 K) before computing the critical time, and decay the initial deficit with the reaeration constant.
• BOD₅ is not ultimate BOD. Back-calculate L₀ first, and use the temperature coefficient for the range you're in.
• TMDL bookkeeping — take the margin of safety once; when it is a percentage, it's a percentage of the TMDL itself, not of the load left after the other allocations. Natural background sits inside the load allocation; a WLA is a permit share, not the current discharge; an implicit margin reserves no load.
• Flow at the right point — use the design flow where the target applies (the stem says where), net of diversions and including effluent that enters above that point; 5.39 goes with cfs, 8.34 with MGD.
• Averaging time — carcinogens average over a lifetime, noncarcinogens over the exposure duration; convert µg/L to mg/L.
• Point of departure — a reference dose is the NOAEL (when the study gives one, not the LOAEL) divided by the uncertainty factors the problem states, and no others.

Have these at your fingertips

The Streeter–Phelps relations, the BOD and temperature-correction relations, the DO saturation table, the reaeration equations (they sit side by side — pick the one the problem names), and the intake relation — plus where they live in the Handbook so you're navigating, not hunting.

Going deeper

• Analysis and Design — the mass balances this area runs on
• Groundwater and Wells — where sorbed contaminants travel

Part of the Water Resources & Environmental exam. Practice this area with exam-real problems — the tutor walks any one you miss. → Start practicing free — free with an account.