3.1 Surface Water Sources, Hydrology & Reservoir Dynamics

3.1 Surface Water Sources, Hydrology & Reservoir Dynamics

Water treatment begins with a comprehensive understanding of raw water sources, their hydrologic behavior, seasonal water quality variations, and physical protection standards. In New Jersey, water system operators manage distinct source waters that reflect the state's diverse hydrogeology.

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Hydrogeology and Water Resources of New Jersey

New Jersey is divided into two distinct hydrogeological regions separated by the Fall Line, an ancient geological boundary that runs diagonally across the state from Trenton northeastward toward Woodbridge.

North of Fall Line (Valley & Ridge, Highlands, Piedmont)
├── Geology: Dense igneous, metamorphic, and consolidated sedimentary rock
├── Primary Supply: Surface water reservoirs and major river basins (>75%)
└── Key Resources: Passaic River Basin, Highlands Watersheds, Delaware River

South of Fall Line (Atlantic Coastal Plain)
├── Geology: Unconsolidated, wedge-shaped seaward-dipping sand, gravel, and clay
├── Primary Supply: Groundwater aquifers (>75%)
└── Key Resources: Kirkwood-Cohansey (unconfined), Potomac-Raritan-Magothy (confined)

Northern New Jersey Surface Water Systems

North of the Fall Line, low-porosity bedrock yields modest groundwater, making surface water the lifeblood of regional supply. Over 75% of drinking water in northern New Jersey originates from surface sources:

• Passaic River Basin: Spans over 935 square miles and includes the Upper Passaic, Pompton, Ramapo, Pequannock, Rockaway, and Whippany rivers. High population densities and industrial history in this basin demand rigorous monitoring for organic matter, ammonia, turbidity, and chemical spills.
• Highlands Reservoirs: Formed by high-quality forested watersheds protected under the New Jersey Highlands Water Protection and Planning Act. Premier reservoir systems include:
  • Wanaque and Monksville Reservoirs: Operated by the North Jersey District Water Supply Commission (NJDWSC), providing over 200 million gallons per day (MGD) to metropolitan northeastern counties.
  • Oradell, Woodcliff Lake, and Lake Tappan: Operated by Veolia Water New Jersey in the Hackensack River basin.
  • Newark Watershed Reservoirs: Charlotteburg, Pequannock, Canistear, Clinton, and Oak Ridge reservoirs nestled in the Highlands.
  • Jersey City System: Boonton and Splitrock reservoirs in the Rockaway River basin.
• Central NJ Pumped Storage Systems: Round Valley Reservoir (55 billion gallons) and Spruce Run Reservoir (11 billion gallons), operated by the New Jersey Water Supply Authority (NJWSA) to augment Raritan River basin flows and support downstream municipal intakes.
• Delaware River: Major surface supply along the western border, serving major municipal treatment facilities such as New Jersey American Water's Delaware River Regional Water Treatment Plant in Delran.

Southern New Jersey Coastal Plain Aquifer Systems

South of the Fall Line, the Atlantic Coastal Plain consists of a thick, seaward-dipping wedge of unconsolidated sedimentary strata containing vast groundwater reserves:

• Kirkwood-Cohansey Aquifer System: A vast, shallow, unconfined (water table) aquifer spanning approximately 3,000 square miles across the Pine Barrens and southern counties. Composed of highly permeable quartz sands and fine gravels with high hydraulic conductivity. Because the water table sits near the surface and recharge occurs directly overhead, the Kirkwood-Cohansey is exceptionally vulnerable to anthropogenic contamination (nitrates, volatile organic compounds, and PFAS). Naturally, its water is unbuffered, acidic (pH 4.5 to 5.5), low in total dissolved solids (TDS), and prone to dissolved iron.
• Potomac-Raritan-Magothy (PRM) Aquifer System: A deep, multi-unit confined aquifer system (Upper, Middle, and Lower sand layers separated by dense confining clay and silt layers) bordering the Delaware River corridor from Mercer County south to Salem County. Decades of heavy municipal and industrial extraction created deep regional cones of depression where potentiometric water levels dropped more than 100 feet below sea level. This threatened saltwater intrusion from Delaware Bay and upward migration of mineralized brackish water. In response, the New Jersey Department of Environmental Protection (NJDEP) designated Water Emergency Critical Areas 1 and 2 under the New Jersey Water Supply Management Act (N.J.S.A. 58:1A), legally capping PRM withdrawals and requiring utilities to develop alternative surface water or desalted sources.

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Surface Water Characteristics & Quality Dynamics

Surface water quality changes continuously in response to precipitation, runoff events, ambient temperature, sunlight, and biological activity.

Physical and Chemical Parameters

Algae Blooms & Harmful Cyanobacteria (HABs)

During warm, sunny summer and early autumn periods, nutrient-rich (phosphorus and nitrogen) surface water bodies often experience massive algal proliferation, including Harmful Algal Blooms (HABs) dominated by cyanobacteria (blue-green algae such as Microcystis, Dolichospermum [formerly Anabaena], Aphanizomenon, and Planktothrix).

Cyanobacteria & Treatment Challenges
├── Cyanotoxins
│   ├── Hepatotoxins: Microcystins, Cylindrospermopsin (damage liver tissue)
│   └── Neurotoxins: Anatoxin-a, Saxitoxins (disrupt nervous system)
├── Taste & Odor Metabolites
│   ├── Geosmin (earthy odor; detection threshold 5–10 ng/L)
│   └── 2-Methylisoborneol / MIB (musty odor; detection threshold 5–10 ng/L)
└── Diurnal pH Dynamics
    ├── Daylight: 6 CO₂ + 6 H₂O → C₆H₁₂O₆ + 6 O₂ (consumes CO₂, pH climbs to 8.5–9.5)
    └── Darkness: C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O (releases CO₂, pH drops to 7.0–7.4)

1. Cyanotoxins: Cyanobacteria synthesize dangerous intracellular toxins that release into the water when cells lyse. NJDEP enforces drinking water health advisories (e.g., 0.3 µg/L microcystins for infants and children under 6; 1.6 µg/L for older children and adults). Conventional coagulation removes intact algal cells, but lysing cells with pre-oxidants (like chlorine) can release dissolved toxins that require powdered activated carbon (PAC), ozonation, or advanced oxidation.
2. Taste and Odor Compounds: Cyanobacteria produce secondary metabolites Geosmin (trans-1,10-dimethyl-trans-9-decalol) and 2-Methylisoborneol (MIB). The human olfactory threshold for both compounds is extraordinarily low: 5 to 10 ng/L (nanograms per liter, or parts per trillion). Conventional alum coagulation, sedimentation, and granular filtration do not remove dissolved geosmin and MIB. Treatment requires feed of powdered activated carbon (PAC at 10 to 30 mg/L), granular activated carbon (GAC) contactors, or ozone.
3. Diurnal pH Fluctuations: Heavy algal growth alters carbonate equilibria. In daylight, intense photosynthesis consumes dissolved carbonic acid and free carbon dioxide ($CO_2$), driving surface water pH upward to 8.5 to 9.5. At night, respiration ceases $CO_2$ uptake and releases $CO_2$, lowering pH back down to 7.0–7.4. Operators must monitor pH continuously; a coagulant dose calibrated for midday raw water at pH 9.0 will fail at night when pH drops.

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Reservoir Thermal Stratification & Seasonal Turnover

In temperate zones like New Jersey, deep lakes and storage reservoirs (>20 to 30 feet deep) exhibit distinct seasonal density layering based on water temperature.

Summer Thermal Stratification Profile
┌──────────────────────────────────────────────────────────────┐
│ EPILIMNION (Upper Layer: 20°C–26°C, High DO, Wind-Mixed)     │
├──────────────────────────────────────────────────────────────┤
│ METALIMNION / THERMOCLINE (Drop of ≥1°C per meter depth)     │
├──────────────────────────────────────────────────────────────┤
│ HYPOLIMNION (Bottom Layer: 4°C–10°C, Stagnant, DO = 0 mg/L)  │
│ • Insoluble Fe³⁺ / Mn⁴⁺ in sediment reduce to soluble Fe²⁺/Mn²⁺│
│ • Sulfate reduction produces H₂S (rotten egg odor)          │
└──────────────────────────────────────────────────────────────┘

Summer Stratification Structure

Water reaches its maximum physical density at 3.98°C (~4°C). As solar radiation warms surface waters in late spring, three vertical zones form:

1. Epilimnion: The warm, light, circulating upper layer (temperatures 20°C to 28°C). Atmospheric contact and photosynthetic activity keep dissolved oxygen near or above saturation (8 to 10 mg/L).
2. Metalimnion (Thermocline): The intermediate transition zone characterized by a steep vertical temperature gradient—defined as a temperature decrease of at least 1°C per meter of depth (or ~0.5°F per foot). It acts as a dense physical barrier that prevents vertical mixing between upper and lower waters.
3. Hypolimnion: The cold, dense, stagnant bottom layer (temperatures 4°C to 10°C). Completely isolated from atmospheric reaeration and solar penetration.

Hypolimnetic Anoxia and Chemical Reduction

As dead algae, organic debris, and plant matter sink into the hypolimnion, benthic aerobic bacteria consume dissolved oxygen during decomposition. Because the thermocline blocks replenishment, the hypolimnion becomes completely anoxic ($\text{DO} = 0.0\text{ mg/L}$).

Once oxygen is depleted, anaerobic microbes utilize alternative electron acceptors, causing the oxidation-reduction potential (redox potential, $E_h$) to drop sharply:

• Iron Reduction: Insoluble ferric iron ($\text{Fe}^{3+}$, as ferric hydroxide $\text{Fe(OH)}_3$ in benthic muds) is chemically reduced to soluble ferrous iron ($\text{Fe}^{2+}$).
• Manganese Reduction: Insoluble manganese dioxide ($\text{MnO}_2$, $\text{Mn}^{4+}$) is reduced to soluble manganous manganese ($\text{Mn}^{2+}$).
• Sulfate Reduction: Obligate anaerobes reduce sulfate ($\text{SO}_4^{2-}$) to dissolved hydrogen sulfide ($\text{H}_2\text{S}$), imparting a noxious "rotten egg" odor and high oxidant demand.
• Ammonia and Methane: Deamination of nitrogenous organics produces elevated free ammonia ($\text{NH}_3$), while methanogenic archaea produce methane ($\text{CH}_4$).

Fall Turnover Mechanics and Operational Crisis

In autumn (typically October or November in New Jersey), ambient air temperatures fall, cooling the epilimnion. As the surface water cools toward 4°C, its density increases until it becomes heavier than the underlying thermocline. Wind turbulence easily breaks down the weakened density gradient, initiating fall turnover (complete vertical destratification).

Fall Destratification Impact on Water Treatment Plants:
├── Instantaneous redistribution of anoxic, reduced water throughout the water column
├── Soluble Fe²⁺ and Mn²⁺ enter the plant intake (causing severe color complaints)
├── Huge surge in chlorine demand caused by H₂S, Fe²⁺, Mn²⁺, and NH₃
└── Coagulation collapse if oxidant and coagulant dosages are not adjusted immediately

Operational Response: Deep reservoir intake towers feature multi-level withdrawal ports (sluice gates). During summer stratification, operators draw water from upper epilimnetic levels to avoid hypolimnetic metals and sulfides. When fall turnover occurs, operators must immediately: (1) start or increase pre-oxidant feeds (potassium permanganate $\text{KMnO}_4$, chlorine dioxide $\text{ClO}_2$, or ozone) to convert soluble $\text{Fe}^{2+}$ and $\text{Mn}^{2+}$ back into insoluble filterable particulates; (2) increase coagulant doses based on updated jar tests; and (3) apply PAC for taste and odor control.