Southeast Hamilton Sewer Study Preliminary Engineering Report - - PowerPoint PPT Presentation

City of Hamilton July 14, 2020

Southeast Hamilton Sewer Study

Preliminary Engineering Report Overview

Preparation Team

WGM Group City of Hamilton

Goals and Objectives

• Identify existing nutrient loads from on site wastewater disposal in

SE Hamilton Area – aquifer and Bitterroot River

• Consider population projections and future wastewater generation
• Evaluate alternative for wastewater treatment that will protect

future water quality – public health & environmental

• Provide document for future wastewater planning within the SE

Hamilton Area

Project Process

• Follow Standard Preliminary Engineering Report Outline
• Provide notice to state and federal agencies for comments
• Complete an Uniform Environmental Checklist
• Evaluate various alternatives
• Prepare cost estimates
• Identify Preferred Alternative

Study Area

• Within the 2006 WWFP Planning

Area

• 1,200 Total Acres
• 1,002 Current Parcels
• Current Population = 2,505
• 13 Different Subbasin Collection

Areas

• 2 Outlying Subbasin Areas
• Golf Course Road
• Grantsdale

Existing Conditions

• No Regional Public Sewer or Water Infrastructure
• Individual Wells (+1,000)
• Individual Septic Systems – Some of Unknown

Condition (+1,000)

• Seepage Pits, Septic Tanks, Drainfields, Others
• Upgradient of City of Hamilton Drinking Water Wells

Need for Project

• Public Health – Nitrates in Aquifer
• Health Effects - Methemoglobeinemia (Blue Baby Syndrome)
• Sensitive Populations – Infants, Pregnant Women, Dialysis Patients
• MCL = 10 mg/L (US EPA)
• Environmental Health – Bitterroot River Nutrient Loading
• Increased Algae Growth
• Decreased Oxygen
• Fish Fatalities

Nutrient Loading Model

• ArcNLET – GIS based model
• Uses topography, soils, septic system locations
• Septic System Ground Water Bitterroot River
• Benefits:
• Relatively quick and inexpensive model to run
• Easy identification of locations with high nutrient loading
• Planning level nutrient estimation tool

Existing Conditions Septic Plume

Projected Conditions Septic Plume Continued Onsite Septic Systems

Shady Ave Area Daly Ave Area Westana Mobile Home Park

Well No. 2 Well No. 1 Well No. 5

Nutrient Loading Model Results

• High density housing on septic systems results in significant nutrient

loads to groundwater wells and Bitterroot River

• Aquifer Nitrogen Loading: 16,700 lbs/year 21,000 lbs/year
• Bitterroot River Nitrogen Loading: 11,000 lbs/year 13,600 lbs/year
• Daly Area: High groundwater impact, greater distance to river
• Shady Area: High groundwater and river impact
• Grantsdale Rd/Westana mobile park: remote location

Population Trends

• Parcels ≥ 5 acres – high

potential density development

• Parcels ≤ 1 acre – low potential

density development

• Parcel Size ~ Projected %

Development

• % Development = Total Future

Lots

• Future Population = 2.5

People/Unit

Population Trends

Population Projections

Wastewater Forecasting

• 85 gallons/capita/day

(2006 Hamilton Wastewater Facility Master Plan)

• Peaking Factor for

Conservative Estimate

• Peak Flows Sensitivity

– Storm and Sewer Analysis Application (SSA)

Future Scenarios Bitterroot River Nutrient Loading

Sewer Master Plan

• Interceptor Mains

(Backbone) – SSA for Pipe Sizing

• Collections Mains for Side

Streets

• Potential for Phased

Development – Decentralized Treatment

• Considerations for

Development Outside of Sewer Study Area

Cost Estimate Example

Daly Interceptor – Detailed Cost Breakdown

Cost Estimate Example

Focus Areas for Immediate Improvements

Benefits of the Preferred Alternative

• Improves public health by reducing sewage-related pollutants

discharged to aquifer providing safe and reliable drinking water.

• Provides protection to the Bitterroot River by reducing nutrient

loading that reach surface water.

• Provides improved opportunity for economic development.
• Provides a resource for future wastewater planning in the SE

Hamilton Area.

THANK YOU!