14.2 Inorganic, Nutrients & Metals Analysis

Nitrate and Nitrite Monitoring, Toxicology & Laboratory Methods

Inorganic nitrogen compounds enter surface and groundwater resources through agricultural synthetic fertilizer runoff, animal feedlot waste, municipal wastewater effluent, and failing septic absorption fields. Under the Safe Drinking Water Act (SDWA), the EPA enforces strict National Primary Drinking Water Regulations for both Nitrate ($\text{NO}_3^-$) and Nitrite ($\text{NO}_2^-$).

Statutory Maximum Contaminant Levels (MCLs)

• Nitrate ($\text{NO}_3^-$): Maximum Contaminant Level = $10\text{ mg/L}$ as Nitrogen ($N$) [equivalent to $45\text{ mg/L}$ when reported as total nitrate molecular weight, $\text{NO}_3$].
• Nitrite ($\text{NO}_2^-$): Maximum Contaminant Level = $1.0\text{ mg/L}$ as Nitrogen ($N$) [equivalent to $3.3\text{ mg/L}$ reported as $\text{NO}_2$].
• Total Nitrate + Nitrite: Maximum Contaminant Level = $10\text{ mg/L}$ as Nitrogen ($N$).

Toxicological Mechanism: Infant Methemoglobinemia

Unlike chronic contaminants whose exposure limits are formulated around multi-decade lifetime cancer risks, nitrate and nitrite represent acute toxic health hazards to infants under six months of age:

1. Gastrointestinal Conversion: In infants under 6 months, the gastric pH is significantly higher (less acidic) than that of older children and adults. This neutral gastric environment permits nitrate-reducing bacteria to proliferate in the upper gastrointestinal tract, rapidly reducing ingested nitrate ($\text{NO}_3^-$) to nitrite ($\text{NO}_2^-$).
2. Hemoglobin Oxidation: Absorbed nitrite enters the bloodstream and directly oxidizes the ferrous iron ($Fe^{2+}$) within hemoglobin into ferric iron ($Fe^{3+}$), converting normal oxygen-carrying hemoglobin into methemoglobin:

$\text{Hemoglobin}(Fe^{2+}) + \text{NO}_2^- \rightarrow \text{Methemoglobin}(Fe^{3+})$

1. Tissue Asphyxiation: Methemoglobin is incapable of binding or releasing dissolved oxygen to cellular tissues. When blood methemoglobin levels exceed 10% to 20%, the infant displays clinical cyanosis, marked by a slate-blue skin discoloration around the lips, ears, and fingernail beds—a life-threatening medical emergency known as infant methemoglobinemia or "blue baby syndrome".
2. Regulatory Notification Tier: An exceedance of the 10 mg/L nitrate MCL requires Tier 1 Public Notification. The utility must alert the public, local public health officials, and pediatric health providers within 24 hours via broadcast media, door-to-door hand delivery, or electronic alert systems, with explicit warnings that boiling water is strictly prohibited because boiling evaporates water and concentrates the non-volatile nitrate.

Laboratory Analytical Methodologies for Nitrogen Compounds

• Cadmium Reduction Method (EPA Method 353.2 / Standard Method 4500-$\text{NO}_3^-$ E):
  • The standard colorimetric benchtop technique. The water sample is passed through an open glass column packed with copper-stabilized granulated cadmium ($Cd$) metal.
  • The cadmium chemically reduces all nitrate in the sample down to nitrite:

$\text{NO}_3^- + Cd + 2 H^+ \rightarrow \text{NO}_2^- + Cd^{2+} + H_2O$

• The resulting nitrite (combining reduced nitrate and original nitrite) diazotizes with sulfanilamide in an acidic medium, forming a diazonium salt. This intermediate couples with N-(1-naphthyl)-ethylenediamine dihydrochloride (NEDD) to produce a highly colored, intensely reddish-purple azo dye.
• The absorbance is measured spectrophotometrically at 543 nm. Subtracting a separately analyzed unreduced nitrite concentration from the cadmium column output yields the true nitrate concentration. Safety note: Cadmium is a regulated toxic heavy metal; spent column waste must be disposed of as hazardous waste.
• Automated Hydrazine Reduction: Uses hydrazine sulfate in alkaline solution to reduce nitrate to nitrite in continuous flow analyzers, eliminating solid cadmium handling.
• Ion Chromatography (EPA Method 300.0):
  • An instrumental method that injects filtered water into an anion-exchange analytical column using a carbonate-bicarbonate eluent.
  • Anions are separated based on their individual chromatographic affinities for the resin bed. The effluent passes through an electrolytic chemical suppressor that removes conductive background eluent ions, and each anion is quantified by an inline electrical conductivity detector.
  • Ion chromatography simultaneously quantifies fluoride, chloride, nitrite, nitrate, bromide, and sulfate from a single sample aliquot without hazardous chemical reagents.

---

Fluoride Chemistry, Regulatory Standards & Analytical Protocols

Fluoride is introduced into municipal water supplies either naturally through the dissolution of mineral deposits (such as fluorite, apatite, and cryolite) or deliberately by water utilities through community water fluoridation to prevent dental decay.

Regulatory Framework and Enforceable Standards

• Primary Maximum Contaminant Level (Primary MCL) = $4.0\text{ mg/L}$: Enforceable federal standard designed to protect against severe skeletal fluorosis, an incapacitating bone disease characterized by dense calcification of ligaments, joint stiffness, bone brittleness, and chronic deformity.
• Secondary Maximum Contaminant Level (Secondary SMCL) = $2.0\text{ mg/L}$: Non-enforceable federal aesthetic guideline. If natural fluoride exceeds 2.0 mg/L, utilities must issue a mandatory public notice to customers warning that children under nine years old are at risk of developing dental fluorosis (cosmetic brown staining, mottling, and pitting of permanent tooth enamel).
• Optimal Fluoridation Target = $0.7\text{ mg/L}$: The Department of Health and Human Services (HHS) and CDC standard for community water fluoridation (operating range 0.6 to 0.8 mg/L), providing maximum cavity prevention with minimal cosmetic fluorosis.

Analytical Testing Methodologies

[ Fluoride Analytical Techniques ]
1. Ion Selective Electrode (ISE) -> LaF3 Crystal -> Requires TISAB Buffer (pH 5.0 - 5.5)
2. SPADNS Colorimetric Method    -> Bleaches Zirconium-Dye Lake -> Measured at 570 nm

1. Ion Selective Electrode (ISE) Method (Standard Method 4500-F$^-$ C)

The fluoride ISE utilizes a sensing element constructed from a single crystal of lanthanum fluoride ($LaF_3$) doped with europium ($Eu^{2+}$) to create mobile fluoride ion vacancies. When the electrode is immersed in an aqueous sample, fluoride ions migrate across the crystal face, generating a potential difference relative to an internal silver/silver chloride ($Ag/AgCl$) reference electrode that conforms to the Nernst equation:

$E = E_0 - S \cdot \log[F^-]$

The Critical Role of TISAB (Total Ionic Strength Adjustment Buffer)

Accurate ISE measurements require mixing every sample and standard in an exact 1:1 volume ratio with TISAB. TISAB fulfills three vital physical and chemical functions:

1. Buffers Sample pH between 5.0 and 5.5:
   • In acidic water ($\text{pH} < 5.0$), free fluoride ions bind with hydrogen ions to form undissociated hydrofluoric acid ($HF$) and hydrogen bifluoride ($HF_2^-$). The $LaF_3$ electrode detects only free, ionized fluoride ($F^-$); thus, acidic conditions cause severe false-negative errors.
   • In alkaline water ($\text{pH} > 8.0$), hydroxide ions ($OH^-$) interfere directly with the electrode. Because $OH^-$ possesses nearly the identical ionic charge and ionic radius as $F^-$, the lanthanum crystal cannot distinguish between them, causing severe false-positive errors.
2. Standardizes Total Background Ionic Strength: The electrode senses chemical activity rather than mass concentration. Adding high-concentration background salts in TISAB fixes the activity coefficient so that measured electrical potential correlates directly with concentration.
3. De-complexes Bound Fluoride (Chelation): Polyvalent metal cations—primarily aluminum ($Al^{3+}$) used in coagulation and ferric iron ($Fe^{3+}$)—form strong coordinate complexes with fluoride (such as $AlF_6^{3-}$ and $FeF^{2+}$), masking fluoride from the electrode. TISAB contains CDTA (cyclohexylenediaminetetraacetic acid), an aggressive chelating agent that preferentially sequesters aluminum and iron ions, liberating all complexed fluoride into the free ionized state.

2. SPADNS Colorimetric Spectrophotometric Method

The SPADNS method relies on the chemical reaction between fluoride and a red-colored zirconium dye lake formed by zirconyl chloride and SPADNS reagent (sodium 2-(para-sulfophenylazo)-1,8-dihydroxy-3,6-naphthalene disulfonate). When fluoride is introduced, it strips zirconium from the dye molecule to form a colorless hexafluorozirconate complex ($ZrF_6^{2-}$). This chemical reaction bleaches the red dye lake, decreasing absorbance at 570 nm in direct proportion to fluoride concentration.

---

Dissolved Oxygen (DO) Analysis

Dissolved oxygen monitoring is critical for optimizing raw water aeration, controlling reservoir destratification, and evaluating corrosion potential.

• Winkler Titration (Azide Modification - Standard Method 4500-O C):
  • The absolute classical wet-chemical reference method. The sample is collected in a specialized 300-mL glass BOD bottle with a tapered ground-glass stopper, taking extreme care to prevent air bubble entrapment.
  • Divalent manganese sulfate ($MnSO_4$) and an alkali-iodide-azide reagent ($NaOH + KI + NaN_3$) are added beneath the liquid surface. The dissolved oxygen oxidizes divalent manganese to form a brown, flocculent precipitate of higher-valence manganese hydroxide [$MnO(OH)_2$].
  • Concentrated sulfuric acid ($H_2SO_4$) is added to dissolve the floc. Under acidic conditions, the oxidized manganese oxidizes iodide ($I^-$) ions into free molecular iodine ($I_2$), releasing an amber iodine color chemically equivalent to the original DO concentration.
  • The liberated iodine is titrated with sodium thiosulfate ($Na_2S_2O_3$) to a pale straw color, mixed with starch indicator (turning dark blue), and titrated until the solution transitions from intense blue to completely colorless.
  • Azide Function: Sodium azide ($NaN_3$) destroys nitrite interference ($\text{NO}_2^-$ up to 5 mg/L), which would otherwise cause continuous catalytic oxidation of iodide and produce false-high DO values.
• Luminescent Optical Dissolved Oxygen Sensors (LDO):
  • Modern solid-state sensor technology. A blue LED excites a ruthenium-based luminescent organometallic dye on the sensor face. The dye emits red phosphorescent light. Collisions with dissolved oxygen molecules quench the excitation (luminescence lifetime decay).
  • The phase shift between excitation and emission is measured. Optical DO sensors do not consume oxygen during measurement, require no continuous stirring, have zero electrolyte solutions, and are completely unaffected by sulfide or chemical fouling.

---

Heavy Metals Analysis & Regulatory Protocols

Heavy metals regulated under SDWA include toxic contaminants with Primary MCLs (Arsenic at $0.010\text{ mg/L}$ or 10 ppb; Lead at $0.015\text{ mg/L}$ Action Level; Copper at $1.3\text{ mg/L}$ Action Level) and nuisance secondary aesthetic metals (Iron at $0.3\text{ mg/L}$ SMCL; Manganese at $0.05\text{ mg/L}$ SMCL).

Analytical Instrumentation for Metals

[ Metal Analytical Instrumentation Hierarchy ]
1. Flame Atomic Absorption (FLAA)       -> Range: mg/L (ppm) -> Fe, Mn, Ca, Mg
2. Graphite Furnace AA (GFAA)            -> Range: ug/L (ppb) -> As, Pb, Cd, Se
3. Inductively Coupled Plasma Mass (ICP) -> Range: sub-ppb / ppt -> Multi-element simultaneous

1. Flame Atomic Absorption Spectrophotometry (FLAA): Sample is aspirated through a nebulizer into an air-acetylene flame ($2,300^\circ\text{C}$). Ground-state atoms absorb resonant light emitted from an element-specific hollow cathode lamp (HCL). Detection range: parts-per-million ($mg/L$). Ideal for high-abundance metals like iron, manganese, and hardness minerals.
2. Graphite Furnace Atomic Absorption (GFAA): Sample is pipetted into an electrothermally heated graphite tube within an inert argon atmosphere. Ramps through drying, ashing (charring organic background), and atomization ($2,000\text{-}2,700^\circ\text{C}$) stages. Retains the atomic cloud in the optical path for several seconds, achieving part-per-billion ($\mu g/L$) sensitivity. Historically required for low-level lead and arsenic.
3. Inductively Coupled Plasma Mass Spectrometry (ICP-MS - EPA Method 200.8): Sample is aerosolized into a high-temperature argon plasma torch ($6,000\text{-}10,000\text{ K}$). Complete thermal ionization occurs. Ions are directed into a high-vacuum mass spectrometer that separates ions based on their mass-to-charge ratio ($m/z$). Provides simultaneous multi-element quantification down to parts-per-trillion ($ng/L$).

Preservation Chemistry for Metals

All aqueous samples collected for total metals analysis must be preserved immediately (or within 14 days of collection) by adding concentrated high-purity nitric acid ($HNO_3$) to lower the sample $\text{pH} < 2.0$:

• In neutral or alkaline water, heavy metal ions hydrolyze to form insoluble metal hydroxides or carbonates that precipitate as sludge.
• Dissolved metal cations exhibit high electrostatic attraction to the negatively charged container walls of polyethylene or borosilicate glass bottles. Acidifying to $\text{pH} < 2.0$ saturates container binding sites with hydronium ions ($H^+$), keeping metal cations completely dissolved in solution.
• Properly acidified metal samples have a statutory holding time of 6 months (except mercury, which has a 28-day holding time due to volatilization).

---

Lead and Copper Rule (LCR) Sampling Protocol

Lead and copper enter drinking water primarily through the corrosive leaching of customer household plumbing, lead service lines (LSLs), brass fixtures, and lead solder, rather than from raw water sources.

Action Levels and Regulatory Status

• Lead Action Level: $0.015\text{ mg/L}$ ($15\text{ ppb}$).
• Copper Action Level: $1.3\text{ mg/L}$ ($1,300\text{ ppb}$).
• Compliance is assessed by calculating the 90th percentile value across all targeted consumer tap samples. If the 90th percentile exceeds either Action Level, the utility has not committed a standard MCL violation, but must trigger mandatory operational actions: optimize Corrosion Control Treatment (CCT), execute source water monitoring, initiate public education campaigns within 60 days, and commence lead service line replacement.

Mandatory Stagnation and Collection Protocol

To ensure sampling captures the maximum dissolved metal concentration accumulated in building plumbing, the EPA mandates a rigid protocol:

1. Sample Container: Certified clean, wide-mouth, 1-liter plastic (polyethylene or polypropylene) bottle.
2. Sampling Point: Interior kitchen or bathroom cold water tap routinely used for potable drinking consumption. Outside hose spigots, utility sinks, and commercial outlets are prohibited.
3. Minimum 6-Hour Stagnation Window: The water must stand motionless in the interior plumbing and service connection for a minimum of 6 hours prior to collection. Sampling is typically performed early in the morning or after residents return from work.
4. First-Draw Collection (No Pre-Flushing): The consumer or operator must place the bottle under the faucet before opening the valve. The tap is opened to a moderate, non-splashing flow, filling the 1-liter bottle completely. Pre-flushing the tap hours prior to stagnation is strictly prohibited, as is removing the faucet aerator or cleaning the faucet spout prior to sampling.
5. Preservation: The 1-liter sample is acidified in the laboratory with concentrated $HNO_3$ to $\text{pH} < 2.0$ and allowed to stand for a minimum of 16 hours prior to analysis.

---

Comparative Analytical Tables

Table 14.2.1: Drinking Water Inorganic Anions & Physical Parameters

Table 14.2.2: Heavy Metals Analysis Matrix