Reconciling Conflicting Base Data on a Former Fuel Site
A 2.4-acre former gasoline station at a downtown corner is being converted to a pocket park. The city provides a 2019 ALTA survey, a 2023 drone photogrammetry surface, and GIS utility data. The boundary line differs by 1.2 feet between the ALTA survey and the photogrammetry; spot elevations differ by up to 0.8 feet. GIS shows a 24-inch storm drain crossing the site that the ALTA survey omits. Geotechnical borings find benzene at 12 feet below grade and groundwater at 8 feet. A 42-inch heritage oak stands 6 feet inside the property line. The adjacent 1920s building has step cracks near the shared foundation. The site slopes gently from the street frontage to a rear alley, and the client has not yet authorized a Phase II environmental site assessment. The client wants schematic design in six weeks.
1. Identify the inventory conflicts and data gaps that must be resolved before schematic design, and specify the field methods you would use.
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Not all conflicts carry equal risk. I would sort them by legal and design consequence:
- Boundary: the ALTA survey is the controlling record, but a 1.2-ft discrepancy affects setbacks, easements, and the oak's protection zone. I would commission a boundary retracement by a licensed surveyor and have the oak located by total station from the same control.
- Topography: drone photogrammetry is unreliable under tree canopy and near vertical walls. I would require field verification by total station at building corners, utility structures, the alley, and the oak, then produce a 1-ft contour map with spot elevations.
- Utilities: the missing 24-inch storm drain is a major conflict. I would call for an 811 ticket, utility as-builts, and CCTV or ground-penetrating radar to confirm location, invert, and easement. The design cannot set grades until this is resolved.
- Contamination: benzene at 12 ft with groundwater at 8 ft is a regulatory and construction-risk issue. I would recommend a Phase II ESA with delineation borings, a soil management plan, and coordination with the environmental engineer before any grading concept.
- Structure: the adjacent step cracks warrant a pre-construction condition survey and monitoring points.
I would present a data-confidence map and a decision log so the client sees which assumptions are firm and which are contingent.
2. How should the contamination and groundwater findings shape the site plan and the client communication?
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The findings should drive a design that minimizes disturbance rather than a design that assumes clean soil. Because groundwater is at 8 ft and benzene is at 12 ft, deep excavation for foundations, ponds, or below-grade utilities would trigger dewatering, soil handling, and possibly a larger regulatory action. I would keep finished grades close to existing grades, use raised planters with a clean-soil barrier for edible or intensive planting, and avoid a community garden unless soil testing supports it. Any structure should use a vapor barrier and passive ventilation if occupied.
For the client, I would separate known facts from assumptions and present a range of cost exposure, not a single alarmist number. A short memo would state: the contamination is below the root zone for most ornamental planting; the risk is concentrated in excavation and groundwater handling; a Phase II ESA and soil management plan are needed to price the risk. I would recommend a contingency line and a decision point before design development. The goal is to keep the park program viable while making the environmental liability visible and budgeted.