We tested a mass-balance mechanism that controls lateral spreading of a pool of crude oil, a light non-aqueous phase liquid (LNAPL), to a stable configuration on the water table in glacial outwash. This designated North Pool oil body is located at U.S. Geological Survey (USGS) research site near Bemidji, MN. The hypothesis states that any LNAPL body stops expanding when the rate of oil mass inflow into a downgradient area of the body balances with rates of mass depletion from that area. A periodic slug-test device was developed to provide aquifer parameters near the North Pool. During a 20-min trial test it obtained a transmissivity within 10% of that determined by the USGS with a 45-hr pumping test in the same aquifer. Recent and archived oil samples were analyzed for mobility-controlling fluid properties and chemical compositions. Mass losses based on changes in moles normalized to Pristane in all samples were used to develop relative mass losses and mass remaining over time. The results were used to estimate the 1979 oil release volume and mass based on an oil volume developed by the USGS using 1990-1992 core analyses. The derivative of the first-order decay function for declining total mass provided mass loss rates in the range of reported historical CO2 mass effluxes. Dated oil-body footprint areas were mapped and integrated with the mass depletion rates to obtain mean historical rates per unit area. Historical changes in fluid properties, hydraulic conductivity, water retention parameters, and with oil thicknesses to model LNAPL transmissivities, gradients, were integrated into a Darcy expression for mass inflow rates. During early spreading when the leading edge was advancing, daily mass inflow exceeded mass depletion rates. During slower late-time spreading, stability occurs within a period defined by intersecting mass-gain and mass-loss curves. Testing with individual oil flow-tubes showed that points along the leading edge can stabilize many years apart. When the LNAPL is unconfined in an effectively homogeneous media, time-dependent changes with LNAPL thickness, transmissivity, mass spreading, and mass depletion are approximated by first-order decay functions. LNAPL body stability testing is recommended at other sites with different LNAPLs and hydrogeologic conditions. Defensible predictions of when and where stability occurs can support LNAPL site management decisions and evolving regulatory policies.