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Equation 1.            ω = ^{(K_HRX π r_HRX2 i_HRX)} ⁄ _{(KA bA iA)}

Equation 1.            w_ave = ^{(K_HRX π r_HRX2 i_HRX)} ⁄ _{(KA bA iA)}

Table 1. Potential reactive media types and target groundwater contaminants for an HRX Well

Reactive Media	Potential Target Groundwater Contaminants
Zero valent iron (ZVI) Bimetallics (e.g., ZVI + Pd, Pt, or Ni)	Chlorinated solvents (CVOCs), nitrate, perchlorate, energetics, chromium, arsenic
Granulated activated carbon (GAC) Organosilicates	CVOCs, Poly- and Perfluoroalkyl substances (PFASs), hydrocarbons, halomethanes
Sustained Release Oxidants	CVOCs, 1,4-dioxane, hydrocarbons, polyaromatic hydrocarbons (PAHs), phenolic compounds
Biodegradable particulate organic carbon (e.g., mulch)	CVOCs, nitrate, perchlorate, energetics
Ion exchange resins	PFAS, brines
Phosphates (e.g., apatite)	Lead, uranium, other metals and radionuclides
Limestone, lime, magnesium oxide	Low pH, acid rock drainage
Barium sulfate (barite)	Radium
Iron sulfide	Chromium, high pH
Zeolites	Ammonium, radionuclides, PFAS

w_ave

In all cases, i_HRX < i_A, but for short wells, i_HRX << i_A, and w_ave is small. However, for long wells (several hundred feet or more), the difference between the hydraulic gradients diminishes. When used as a screening calculation, i_HRX and i_A can be assumed to be approximately equal in many cases. By inspection of Equation 1, it is clear that w_ave increases as the permeability contrast between the aquifer and reactive media increases, and