Designing Around Seasonal High Groundwater Jennifer Deal, Graeme Bowles, Don Hullings January 31, 2017

Introduction Seasonal high groundwater elevation affects many landfill design aspects Design elevation Grading and fill requirements Construction dewatering Stormwater management Airspace Monitoring well construction © 2017 Cornerstone. All rights reserved.

Seasonal High Groundwater Determination Site specific Regional Soil survey © 2017 Cornerstone. All rights reserved.

Seasonal High Groundwater Determination Site Specific Seasonal High Goal Data sources Considerations Data outliers Climatic patterns Topography Goal – provide a number with relative certainty considering the data set available and local experience. Data sources local, state or fed databases - series of ROMP wells in several FL locations, long data record, multiple units monitored client data collection - adjacent sites and current development site, prior permitting efforts and hydraulic assessments, current wells and consideration for planning in advance adjacent sites – reporting requirements for multiple reasons, FDEP Occulus, Water Management Districts Considerations Outliers, anomalies, bad data, artificial short term reason, reasonable test with assessment against the trend for the well and the well set, elimination. Climatic patterns – designing in wet vs. dry season, swing of as much as 25 inches annually, “normal” is 53 in Sarasota, long term trend influencing conservative design, NOAA record (go to pic) seasonal cycle – 5 to 7 years, with outliers, magnitude of the bars is relative to amount of precip. Topography – affects how water behaves in the subsurface, shallows at the toe of slopes, mounds at water divides, responds to changes in topography to a degree, reliant on upslope hydraulic pressure, wetland offsets closing out ponds, canals, swales or ditches, level topography will not show the same as historic water table, even pressure not lower outlet (subsurface connections remain). © 2017 Cornerstone. All rights reserved.

Seasonal High Groundwater Determination Climatic patterns – designing in wet vs. dry season, swing of as much as 25 inches annually, “normal” is 53 in Sarasota, long term trend influencing conservative design, NOAA record (go to pic) seasonal cycle – 5 to 7 years, with outliers, magnitude of the bars is relative to amount of precip. Palmer Drought Severity Index - NOAA © 2017 Cornerstone. All rights reserved.

Seasonal High Groundwater Determination Regional Relative comparison Setting History © 2017 Cornerstone. All rights reserved.

Seasonal High Groundwater Determination Soil survey Original intent – soil conservation Focus on flora and fauna Vadose zone Hydric vs. non-hydric Predictions for uses of soils other than farming, grazing, wildlife habitat, and forestry have tended to concentrate on limitations of soils for the intended uses. Not intended to provide water table information. Design purposes © 2017 Cornerstone. All rights reserved.

Example – St. Marks C&D Facility © 2017 Cornerstone. All rights reserved.

Example – St. Marks C&D Facility Shallow wells on-site but no record of data Several years of data for adjacent site Compared lithology from Nine Mile and St. Marks to confirm consistency between the sites Determined seasonal high at Nine Mile relative to ground surface and applied to St. Marks © 2017 Cornerstone. All rights reserved.

Example – St. Marks C&D Facility Seasonal high one-foot below ground surface Construction below seasonal high Earthfill Permittable FAC 62-701.400(11) “equivalent degree of protection for the environment” © 2017 Cornerstone. All rights reserved.

Example – St. Marks C&D Facility Additional 60-mil geomembrane and GCL Preliminary approval from FDEP, application currently under review © 2017 Cornerstone. All rights reserved.

Expense of Raising Grades One foot of earthfill per acre is 1613 CY/ac At $3/CY = $4840/ac At $10/CY = $16,130/ac Value of lost airspace At $5/CY = $8067/ac I think we are know about federal subtitle regs and how they are adopted by individual states. Post-closure care is required and while most people plan for 30 years,, it can be shorter although unlikely, but quite possibly longer. How much longer can be determined by the state on an individual basis © 2017 Cornerstone. All rights reserved.

Costs for Going Below SHGWT One foot: Assume no dewatering as “actual” groundwater is more than a foot above SHGWT Cost of placing additional layer of geomembrane below GCL to address effects of groundwater per FAC 62-701 1600 sf (40x40) x $0.60/sf = $1,000 Cost to convince FDEP = Priceless Subgrade SHGWT Groundwater Level at time of construction © 2017 Cornerstone. All rights reserved.

Costs for Going Below SHGWT Two feet: Two well points = $1000 Header pipe = $1000 Pump rental for several months = $1000 Pump operations = $4000 ($40/day for 100 days) Added Geomembrane = $1000 Dewatering Permit (simple recharge) = $2000 Say $10,000 Subgrade SHGWT Groundwater Level at time of construction © 2017 Cornerstone. All rights reserved.

Costs for Going Below SWGWT Subgrade at SWGWT 8 ft Ditch grade sump Isolates area below groundwater to ditch and sump Dewater effectively to 8+ at sump Achieve an average of 4 feet or earthfill/capacity Plan view © 2017 Cornerstone. All rights reserved.

Cost For Going Below SHGWT Four feet: Extensive Dewatering = $75,000 (check out YouTube) Geomembrane for sump and ditch = $25,000 Extra Gravel = $100,000 Dewatering Permit = $20,000 Say $220,000 © 2017 Cornerstone. All rights reserved.

Cost and Benefits of Dewatering © 2017 Cornerstone. All rights reserved.

Questions? © 2017 Cornerstone. All rights reserved.