Stormwater Infiltration Testing in Lower Saucon Township: What Your Project Actually Needs

Most stormwater problems on a development project don’t start in the field. They start on paper, months earlier, when someone assumed the ground would take water and nobody tested it properly. By the time that assumption fails, the plan has already been designed around it — and redesigning a stormwater system after a township review is expensive in a way that testing never is.

If you’re building in Lower Saucon Township — a new home, a subdivision, a commercial site — infiltration testing is where your stormwater plan begins. Here’s what’s required, why the geology here makes it more complicated than most places, and why who does the work matters as much as whether it gets done.

When Stormwater Testing Is Triggered

Two layers of regulation apply to earth disturbance in Pennsylvania, and they stack on top of each other.

State — 25 Pa. Code Chapter 102. Any earth disturbance over 5,000 square feet requires a written Erosion and Sediment Control plan. Once a project disturbs one acre or more — including projects phased over time, or individual lots that are part of a larger common plan of development or sale — it requires NPDES permit coverage for construction stormwater. Most projects apply under the PAG-02 general permit; sites that can’t meet its eligibility criteria need an individual NPDES permit instead. That threshold catches more projects than people expect, because the acre is measured cumulatively across the whole plan, not lot by lot.

An NPDES application isn’t just an erosion plan. It requires a Post-Construction Stormwater Management plan showing how runoff will be managed permanently, after the equipment leaves — and if that plan relies on infiltration, it has to be backed by field-measured infiltration rates. That’s where testing comes in.

Local — Lower Saucon Township Chapter 137. The Township’s Stormwater Management ordinance applies to regulated earth disturbance activities within its borders, and no work may begin until the Township approves a plan demonstrating compliance. Chapter 137 also requires an approved operations and maintenance plan for any permanent stormwater BMPs before earth disturbance starts — meaning you need to have documented not just how the system will be built, but who maintains it and how, before anyone breaks ground.

Practically, this means two review tracks running at once: the Northampton County Conservation District handling the NPDES side, and the Township handling Chapter 137 and subdivision review. Both need the same underlying soils information, and neither moves until they have it.

Why Lower Saucon Is Harder Than Most Townships

Much of Lower Saucon sits on carbonate bedrock — limestone and dolomite geology. Carbonate rock dissolves over time, which is what produces sinkholes, closed depressions, fracture traces, and the underground voids that make this region interesting to geologists and stressful to developers.

Concentrating stormwater and putting it into the ground above carbonate bedrock is exactly how a stable site becomes an unstable one. The Township’s ordinance reflects that. Chapter 137 includes specific guidance for infiltration BMPs in carbonate bedrock, and the Township’s general requirements prohibit placing stormwater control basins in or over sinkholes, closed depressions, lineaments, fracture traces, caverns, ghost lakes, or disappearing streams. Basins must be set back at least 100 feet from the rim of sinkholes, closed depressions, and disappearing streams, and at least 50 feet from lineaments, fracture traces, and surface or subsurface pinnacles. Stormwater generated by development cannot be directed into any of those features.

Those constraints do two things to a project. First, they can rule out large portions of a site for infiltration before testing even starts. Second, they mean a competent site evaluation isn’t just measuring how fast water moves — it’s identifying geologic features that determine whether infiltration belongs in that location at all. A test that returns a beautiful infiltration rate directly over a fracture trace has told you the most dangerous possible thing: that the ground will happily take your stormwater straight down into rock you don’t want it in.

The Double-Ring Infiltrometer Test — and Why It Isn’t a Perc Test

Homeowners often assume stormwater testing and septic perc testing are the same thing. They aren’t, and the difference matters to your engineer.

A percolation test comes out of Pennsylvania’s Act 537 sewage program. Water in a perc hole moves out through both the bottom and the sides, so the measured rate has to be adjusted to account for that lateral movement. It’s designed for sizing an absorption area for a septic system — a different question than the one stormwater design is asking.

A double-ring infiltrometer test (ASTM D3385) measures the vertical movement of water through the bottom of the test area. Two concentric rings are seated in the soil; the outer ring is flooded alongside the inner ring so that lateral spreading happens outside the measurement zone, and what the inner ring records is closer to true vertical infiltration. That matters because the floor of an infiltration basin is the surface actually doing the infiltrating — so the test needs to measure the same thing the basin will do.

PA DEP’s Stormwater Best Management Practices Manual, Appendix C, lays out the protocol. A few points worth knowing before you schedule:

• Testing happens at the proposed BMP location and depth — not at a convenient corner of the property. Tests must characterize conditions at the bottom elevation of the facility as designed.
• Test pits come first. The soil profile is logged — horizons, depth to seasonal high water table, depth to bedrock, and any limiting zones. Those observations frequently change the design before a single infiltration number is recorded.
• Multiple tests, geometric mean. Design rates are derived from multiple tests using the geometric mean, not the arithmetic average — an important distinction that keeps one optimistic reading from carrying a design.
• Weather and timing matter. Tests shouldn’t be run in the rain, within 24 hours of significant rainfall, or in freezing conditions. Results from a saturated or frozen site aren’t usable.
• Restoration is part of the job. Test pits are backfilled with the original soil and the surface replaced with the original topsoil.

Why the Team Matters More Than the Equipment

Anyone with a machine can dig a hole. Stormwater testing done properly takes three roles working together, and a weak link in any one of them shows up later as a rejected plan or a failed system.

A qualified soil scientist or geologist reads the profile. This is the part that can’t be shortcut. Identifying a seasonal high water table from redoximorphic features, recognizing a fragipan or a restrictive layer, spotting evidence of carbonate solution activity — these are trained judgments, and in this geology they’re the difference between a design that works and one that undermines a building. An inexperienced eye sees dirt; a soil scientist sees a horizon sequence that predicts how the site behaves in a wet spring.

A design engineer takes those field results and turns them into a PCSM plan that satisfies both Chapter 102 and the Township’s ordinance — applying the required factors of safety, siting facilities outside the prohibited setbacks, and sealing the plan that goes to review.

An experienced excavator opens the ground so the other two can do their jobs. That sounds like the simple part. It isn’t. Test pits have to be located to the plan, excavated to the correct depth without smearing or compacting the very surface being tested, cut with walls that can be safely observed under OSHA requirements, kept clean enough to read, and backfilled and restored properly afterward. A pit dug carelessly — smeared sidewalls, a compacted floor, wrong depth, or wrong location — produces numbers that are worse than no numbers, because they look like data.

The projects that go smoothly are the ones where these three show up on the same day, coordinated, with the plan in hand. The projects that drag are the ones where testing was treated as a box to check.

What We Handle

Sullivan Excavating and Landscaping provides the excavation side of stormwater testing across Lehigh and Northampton Counties. We work alongside the soil scientists and engineering firms your project already has — or we can point you toward the ones we work with regularly.

• Test pit excavation located and cut to the design plan and depth
• Pits prepared so soil profiles can be read and observed safely
• Coordination and scheduling with your soil scientist, engineer, and the Township or Conservation District
• Backfill and surface restoration when testing is complete
• Access work where equipment can’t reach a proposed BMP location

Start Earlier Than You Think You Need To

The single most common mistake on these projects is timing. NPDES general permit applications are recommended to reach the Conservation District roughly six months before the anticipated start date, and that clock only starts once the design is complete — which requires the test results, which require a window of workable weather and a coordinated field day.

Add Township review under Chapter 137 running alongside it, and a project that “starts in spring” realistically needs its soils work happening the previous summer or fall. Sites that test in the fall have room to redesign if the results come back poor. Sites that test in April do not.

Planning a Project in Lower Saucon Township?

If you’re working through stormwater requirements for a building permit or subdivision approval, call 570-856-3408. Tell us where the site is and what stage you’re at, and we’ll tell you honestly what the testing will involve and how it fits your schedule — including whether you’re early enough to be comfortable.

This post is general information about the permitting process, not engineering advice or a determination about any specific property. Requirements vary by site and municipal ordinances are periodically amended — confirm current requirements with Lower Saucon Township and the Northampton County Conservation District, and rely on your project’s engineer and soil scientist for design decisions.