Why New Hampshire Well Water Is the Way It Is

The geology that makes NH water challenging. Understanding what's beneath your well helps you understand what's in your water.

The Granite State — Literally

New Hampshire's nickname isn't just marketing. The state is built on some of the oldest and hardest rock in North America — primarily granitic and metamorphic formations that date back hundreds of millions of years.

This granite is beautiful, durable, and the source of nearly every water quality challenge private well owners face. Arsenic, radon, uranium, and fluoride — all come from minerals embedded in the bedrock.

Two Types of Aquifers

NH has two fundamentally different types of aquifers, and which one your well taps determines much about your water quality:

Bedrock Aquifers

Most NH private wells are drilled into fractured bedrock. Water flows through cracks and fractures in the granite and metamorphic rock — not through pore spaces like in sand or gravel. This means:

• Water has extended contact with rock — dissolving minerals including arsenic, uranium, and radon
• Yield varies enormously — your well hits whatever fractures happen to be at that location
• Water quality is highly localized — neighboring wells can penetrate different fractures in different rock types
• Deeper wells generally have higher mineral content due to longer contact time

Glacial (Surficial) Aquifers

NH was entirely covered by glaciers during the last ice age. When the glaciers retreated (~15,000 years ago), they left behind:

• Glacial outwash — thick deposits of sand and gravel, particularly in river valleys. These make excellent aquifers with good water quality.
• Glacial till — unsorted mix of clay, sand, gravel, and boulders. Poor aquifer, but many shallow wells draw from it.
• Lake-bottom sediments — fine silts and clays deposited in glacial lakes. Not good aquifers.

Wells in glacial outwash deposits generally have lower arsenic and radon than bedrock wells, because the water hasn't been in prolonged contact with granitic rock. However, they can be more vulnerable to surface contamination.

The Arsenic Connection

NH's arsenic problem is geological, not industrial. The arsenic occurs naturally in several bedrock minerals:

• Arsenopyrite — an iron-arsenic sulfide mineral common in granitic and metamorphic rock
• Arsenic-substituted pyrite — arsenic atoms replacing iron in common pyrite
• Secondary arsenic minerals — formed by weathering of primary minerals

When groundwater flows through fractures containing these minerals, it slowly dissolves arsenic. The concentration depends on: the specific rock type, how long the water has been in contact, the water's pH and chemistry, and the particular fractures your well intersects.

The NH Geological Survey has mapped arsenic probability by town, correlating bedrock type with well test data. Southeastern and central NH tend to have the highest risk, but elevated arsenic can occur anywhere in the state.

Radon: The Radioactive Byproduct

Granite contains trace amounts of uranium, which undergoes radioactive decay. One step in that decay chain produces radon gas — an invisible, odorless, radioactive gas that dissolves readily in groundwater.

NH has some of the highest radon levels in the nation, both in indoor air and in well water. The two are related: the same bedrock that puts radon in your water also puts it in your soil, where it can seep into your basement. See our radon guide.

Glacial Legacy

The glaciers didn't just leave deposits — they shaped the entire landscape:

• Valleys were carved deeper, creating the river valleys where the best surficial aquifers exist
• Bedrock was scoured, exposing fresh rock surfaces that are still releasing minerals into groundwater
• Drainage patterns were disrupted, creating the Lakes Region and countless wetlands
• Soil depths vary wildly — from bare bedrock on hilltops to 100+ feet of deposits in valleys

This glacial legacy explains why water quality can change so dramatically over short distances in NH.