The noncancer risk-based concentration of a chemical in drinking water is a function of the ingestion, inhalation and dermal contact exposures to a hypothetical residential receptor.
Ingestion
The noncancer risk-based concentration for the ingestion of drinking water is calculated using the equation:
(equation 1)
Workbook: MCP GW.xls, sheet: GW-1, column: D
| parameter |
|
description |
|
|
[OHM]ing-nc |
= |
Target noncancer groundwater concentration based on ingestion (µg/L) |
|
HI |
= |
Target Hazard Index level (unitless) |
|
RfD oral |
= |
oral Reference Dose (mg/kg/day) |
|
DWEFing-nc |
= |
Drinking Water Exposure Factor for noncancer, ingestion (L*mg)/(kg*ug*d) |
|
RAForal-nc |
= |
Relative Absorption Factor for noncancer, oral exposure (unitless) |
|
The exposure term, DWEF ing-nc, is a compilation of receptor exposure assumptions that are not chemical-specific. This factor is calculated:
(equation 2)
Workbook: MCP GW.xls, sheet: DW Assumptions, cell J18
| parameter |
|
description |
|
|
DWEFing-nc |
= |
Drinking Water Exposure Factor for noncancer, ingestion (L*mg)/(kg*ug*d) |
|
VI |
= |
Volume of drinking water ingested (liters/day) |
|
EF1 |
= |
Exposure Frequency (days/week) |
|
EF2 |
= |
Exposure Frequency (weeks/year) |
|
EP |
= |
Exposure Period (years) |
|
C1 |
= |
Conversion Factor (mg/µg) |
|
C2 |
= |
Conversion Factor (years/day) |
|
BW |
= |
Body Weight (kg) |
|
AP |
= |
Averaging Period (years) |
|
Dermal
The evaluation of dermal exposures associated with the use of drinking water is based on the U.S. EPA's Draft Dermal Exposure Guidance. For some chemicals, the estimated results fall outside the "Effective Predictive Domain". For these chemicals, the default approach recommended in the DEP's 1995 Guidance for Disposal Site Risk Characterization is used (equation 3b).
(equation 3a)
(equation 3b)
Workbook: MCP GW.xls, sheet: GW-1, column: E
| parameter |
|
description |
|
|
[OHM]derm-nc |
= |
Target noncancer groundwater concentration based on dermal contact (µg/L) |
|
HI |
= |
Target Hazard Index level (unitless) |
|
RfDoral |
= |
oral Reference Dose (mg/kg/day) |
|
DWDFnc |
= |
Drinking water dermal factor, noncancer ((cm2*event)/(kg*day)) |
|
OAEnc |
= |
Oral absorption efficiency, noncancer |
|
DAevent |
= |
Absorbed dose per event, per exposed skin area, per unit concentration ((mg/cm2-event)/(µg/L)) |
|
DWEFing- nc |
= |
Drinking water exposure factor, ingestion, noncancer ((L*mg)/(kg*ug*d)). See Equation 2. |
|
DM |
= |
Dermal multiplier (unitless) |
|
(equation 4)
Workbook: MCP GW.xls, sheet: DW Assumptions, cell V13
| parameter |
|
description |
|
| DWDF |
= |
Drinking water dermal factor, ((cm2*event)/(kg*day)) |
| EF1 |
= |
Exposure Frequency (events/day) |
| EF2 |
= |
Exposure Frequency (days/year) |
| EP |
= |
Exposure Period (years) |
| SA |
= |
Exposed skin surface area (cm2) |
| C1 |
= |
Conversion Factor (years/day) |
| BW |
= |
Body Weight (kg) |
| AP |
= |
Averaging Period (years) |
|
Non-Steady State equation for estimating dermally absorbed dose from organic chemicals in water for t event less than or equal to t*. The non-Steady state equation is used when the event duration (t event) is less than or equal to the time to reach steady state (t*).
(equation 5a)
Workbook: MCP GW.xls, sheet: DW Dermal, column M (columns I x L)
Steady State equation for estimating dermally-absorbed dose from organic chemicals in water for t event greater than t*. The steady state equation is used when the event duration (t event) is greater than the time to reach steady state (t*).
(equation 5b)
Workbook: MCP GW.xls, sheet: DW Dermal, column M (columns I x L)
Equation for estimating dermally-absorbed dose for inorganic chemicals in water
(equation 5c)
Workbook: MCP GW.xls, sheet: DW Dermal, column M (columns I x L)
| parameter |
|
description |
|
| DAevent |
= |
absorbed dose per event per area skin exposed (mg/cm2-event) |
| FA |
= |
Fraction absorbed (unitless) |
| Kp |
= |
sc permeability constant (cm/hr) |
| C |
= |
Conversion Factor (m3/cm3) |
| Cw |
= |
[OHM] in water (set equal to 1 to yield results per unit concentration) (mg/m3) |
| t |
= |
lag time (time for chemical to cross sc) (hrs) |
| tevent |
= |
Event duration (hrs) |
| B |
= |
Ratio of permeability of chemical in sc to permeability of chemical in ve (dimensionless) |
|
Equation for predicting sc permeability constant (Kp) for organic chemicals:
(equation 6)
Workbook: MCP Toxicity.xls, sheet: Toxicity, column BE
| parameter |
|
description |
|
| Kp |
= |
Permeability coefficient (cm/hr) |
| Kow |
= |
octanol/water partition coefficient |
| MW |
= |
Molecular Weight (g/mole) |
|
Equation for calculating ratio of permeability of chemical in strateum corneum to permeability in viable epidermis (B)
(equation 7)
Workbook: MCP GW.xls, sheet: DW Dermal, column B
| parameter |
|
description |
|
| B |
= |
Ratio of permeability of chemical in strateum corneum to permeability of chemical in viable epidermis (dimensionless) |
| Kp |
= |
Permeability coefficient (cm/hr) |
| MW |
= |
Molecular Weight (g/mole) |
|
Equations for calculating time to reach steady state (t*)
When B is less than or equal to 0.6:
(equation 8a)
When B is greater than 0.6:
(equation 8b)
Workbook: MCP GW.xls, sheet: DW Dermal, column E
| parameter |
|
description |
|
| t* |
= |
time to reach steady state (hr) |
| t |
= |
lag time (time for chemical to cross sc) (hrs) |
| b |
= |
empirical variable used to calculate t* |
| c |
= |
empirical variable used to calculate t* |
| lsc |
= |
thickness of skin (cm) |
| Dsc |
= |
effective diffusivity for chemical transfer through the skin |
|
Equations for calculating b and c
(equation 9)
(equation 10)
Workbook: MCP GW.xls, sheet: DW Dermal, columns G and H
| parameter |
|
description |
|
| c |
= |
empirical variable used to calculate t* |
| B |
= |
Ratio of permeability of chemical in sc to permeability of chemical in ve (dimensionless) |
| b |
= |
empirical variable used to calculate t* |
|
Equation for calculating lag time ( t)
(equation 11)
Workbook: MCP GW.xls, sheet: DW Dermal, column C
| parameter |
|
description |
|
| lsc |
= |
thickness of skin (cm) |
| Dsc |
= |
effective diffusivity for chemical transfer through the skin |
|
Equation for calculating effective diffusivity (Dsc)
(equation 12)
Workbook: MCP GW.xls, sheet: DW Dermal, column D
| parameter |
|
description |
|
| Dsc |
= |
effective diffusivity for chemical transfer through the skin |
| MW |
= |
Molecular Weight |
| lsc |
= |
thickness of skin (cm) |
|
Inhalation
Inhalation exposures associated with the use of drinking water in the home for showering are evaluated using the approach described in Foster and Chrostowski (1987).
(equation 13)
Workbook: MCP GW.xls, sheet: GW-1, column F
| parameter |
|
description |
|
| [OHM]inh-nc |
= |
Target noncancer groundwater concentration based on inhalation (µg/L) |
| HI |
= |
Target Hazard Index level (unitless) |
| RfC |
= |
inhalation Reference Concentration (mg/m3) |
| C1 |
= |
Conversion Factor (µg/mg) |
| EXPinh-nc |
= |
Inhalation Exposure Factor, noncancer ((µg/m3)/(µg/l)) |
|
(equation 14)
Workbook: MCP GW.xls, sheet: DW Inhalation, column J
| parameter |
|
description |
|
| EXPinh |
= |
Inhalation Exposure Factor ((µg/m3)/(µg/l)) |
| S |
= |
Indoor air generation rate (µg/(m3-min)) |
| Rae |
= |
Air Exchange Rate (1/min) |
| Ds |
= |
Shower Duration (minutes) |
| Dt |
= |
Total time in shower room (minutes) |
| n |
= |
Number of showers per day (unitless) |
| EF1 |
= |
Exposure Frequency (days/week) |
| EF2 |
= |
Exposure Frequency (weeks/year) |
| EP |
|
Exposure Period (years) |
| C3 |
|
Conversion Factor (years/minute) |
| AP |
= |
Averaging Period (years) |
|
(equation 15)
Workbook: MCP GW.xls, sheet: DW Inhalation, column I
| parameter |
|
description |
|
| S |
= |
Indoor air generation rate (µg/(m3-min)) |
| Cwd |
= |
Concentration leaving water droplet (µg/l) |
| FR |
= |
Shower flow rate (l/minute) |
| SV |
= |
Shower room air volume (m3) |
|
(equation 16)
Workbook: MCP GW.xls, sheet: DW Inhalation, column H
| parameter |
|
description |
|
| Cwd |
= |
Concentration leaving water droplet (µg/l) |
| Cw0 |
= |
Shower water concentration, set equal to 1 for standard derivation (µg/l) |
| KaL |
= |
Adjusted mass transfer coefficient (cm/hr) |
| ts |
= |
Shower droplet time (seconds) |
| 60d |
= |
the specific interfacial area, 6/d, for a spherical droplet of diameter d (mm), multiplied by conversion factors, hr/3600 sec and 10 mm/cm |
|
(equation 17)
Workbook: MCP GW.xls, sheet: DW Inhalation, column G
| parameter |
|
description |
|
| K aL |
= |
Adjusted mass transfer coefficient (cm/hr) |
| KL |
= |
Overall mass transfer coefficient (cm/hr) |
| Tl |
= |
Calibration water temperature of KL ( °K) |
| µs |
= |
Water viscosity at Ts (cp) |
| Ts |
= |
Shower water temperature ( °K) |
| µl |
= |
Water viscosity at Tl (cp) |
|
(equation 18)
Workbook: MCP GW.xls, sheet: DW Inhalation, column F
| parameter |
|
description |
|
| KL |
= |
Overall mass transfer coefficient (cm/hr) |
| kl |
= |
Liquid film mass transfer coefficient (cm/hr) |
| R |
= |
Universal Gas Constant (atm*m3/mol* °K) |
| T |
= |
Air temperature ( °K) |
| HLC |
= |
Henry's Law Constant (atm*m3/mol) |
| kg |
= |
Gas-film mass transfer coefficient (cm/hr) |
|
(equation 19)
Workbook: MCP GW.xls, sheet: DW Inhalation, column E
| parameter |
|
description |
|
| kl |
= |
Liquid film mass transfer coefficient (cm/hr) |
| kl(CO2) |
= |
Liquid-film mass transfer coefficient, CO2 (cm/hr) |
| 44 |
= |
Molecular weight of CO2 (g/mole) |
| MWvoc |
= |
Molecular Weight of contaminant (g/mole) |
|
(equation 20)
Workbook: MCP GW.xls, sheet: DW Inhalation, column D
| parameter |
|
description |
|
| kg |
= |
Gas-film mass transfer coefficient (cm/hr) |
| kg(H2O) |
= |
Gas-film mass transfer coefficient, water (cm/hr) |
| 18 |
= |
Molecular weight of water (g/mole) |
| MWvoc |
= |
Molecular Weight of contaminant (g/mole) |
|
Cumulative Noncancer Risk-Based Concentration
Having calculated the target groundwater concentrations, for ingestion, dermal contact and inhalation separately, a target groundwater concentration protective of all three exposure pathways is calculated using a harmonic mean:
(equation 21)
Workbook: MCP GW.xls, sheet: GW-1, column G
| parameter |
|
description |
|
| [OHM]gw-nc |
= |
Target noncancer risk-based concentration in groundwater (µg/L) |
| [OHM]ing-nc |
= |
Target noncancer groundwater concentration based on ingestion (µg/L) |
| [OHM]dermal-nc |
= |
Target noncancer groundwater concentration based on dermal contact (µg/L) |
| [OHM]inh-nc |
= |
Target noncancer groundwater concentration based on inhalation (µg/L) |
|
priorities & results
laws & rules
