Twin Cities aggregate resources report

Report abstract

Past geologic events have provided the seven-county metropolitan area with large deposits of sand and gravel as well as dolomitic bedrock, both of which are used for construction aggregates. Prior to European settlement in about 1840 the metropolitan area contained more than 5.7 billion tons of aggregate resources, based on the current industry definition of economically viable sites. Between 1840 and 1997 the metropolitan area has grown to include 972,000 homes that shelter more than 2.5 million people. Businesses and government have also grown, and now employ nearly 1.5 million people. In 1997 more than 970 of the 2975 square miles that make up the metropolitan area were urbanized.

The urbanization process has involved the mining of substantial amounts of aggregate to build roads, shopping malls, businesses, and factories, homes, and recreation areas. More than 140 square miles of land containing good aggregate have been paved over or otherwise excluded from use. As a result, only 1.7 billion tons of aggregate remained available in the metropolitan area in 1997. Thus, in the 160 years since European settlement 70 percent of the aggregate resources were consumed or rendered unavailable. The 1.7 billion tons of aggregate resources that are estimated to remain may well overstate the amount of material that could be mined or marketed. The estimate does not take into account the impacts of future zoning decisions, or the willingness of landowners to allow aggregate mining. These unquantifiable factors could reduce the amount of aggregate available to an amount significantly less than 1.7 billion tons.

In its 1996 Regional Blueprint, the Metropolitan Council outlined its regional growth strategy, and the areas that it anticipates being urbanized by 2020 and 2040. It anticipates that an additional 1300 and 1560 square miles of land, respectively, will be urbanized by those dates. Neither the regional or local plans provide mechanisms for the preservation of aggregate to ensure that the resources are mined prior to urbanization. Thus it is anticipated that the region will lose almost 600 million tons or 35 percent of the remaining 1997 resource base through the paving over of deposits, or the fragmentation of deposits into parcels too small to mine.

Demand for aggregate in the metropolitan area will continue to grow. In 1998 the annual demand for new aggregate was about 27 million tons. The forecasted demand for 2040 shows a range of values between 41 and 58 million tons, depending on whether one projects on the basis of the long-term usage trend (1950-98) or on the basis of the shorter-term, high-demand trend of the 1990-98 period. Regardless, between 1.4 billion and 1.8 billion tons of aggregate will be needed between 2000 and 2040.

Aggregate resources of the metropolitan area could be exhausted as early as 2028, because of the loss of aggregate-bearing land to urbanization together with the increased demand, which will be between 2 billion and 2.4 billion tons. Planning and regulatory measures that would ensure that aggregate resources are extracted prior to urbanization of an aggregate-rich site would extend the availability of the resources by 10 to 15 years. Potential land-use conflicts also exist on about 17 square miles of land, where aggregate-bearing land is enrolled in the Minnesota Agricultural Preserves Program.

A 1983 study of aggregate resources by the Metropolitan Council stated that "The delivered price of aggregate is affected significantly by transportation costs...the delivered price of the aggregate doubles at approximately 19 miles... ." While the present study did not look at the cost of aggregate, aggregate providers state that the delivered price of aggregate doubles when transported 15 miles. As the resources of the Metropolitan Area become depleted, aggregate needed for urban development will have to be imported from outside the seven-county metropolitan area. The result will be a significant increase in the cost of urban development, particularly office buildings, roadways and bridges, all of which use significant amounts of aggregate.

Report methods

Digital land-use maps were overlaid on the digital geologic map (MGS Miscellaneous Map Series M-102). Areas of aggregate-bearing lands that are not affected by land uses that rule out aggregate mining (unencumbered aggregate-bearing land) were then calculated. A geographic information system (GIS) procedure similar to that used for estimating the geological endowment was used to calculate the tonnage of aggregate available from the unencumbered aggregate-bearing land. These calculations were based on land-use (urbanization) determined from aerial photographs taken in December 1997, and the predicted urban land-use patterns for the years 2020 and 2040. Additionally, environmentally sensitive areas such as streams, wetlands over five acres, and Scientific and Natural areas were buffered and excluded. The study did not take into account the impacts of future zoning decisions, or the willingness of landowners to allow aggregate mining.

The details of the land-use coverages employed and the GIS procedures by which the unencumbered aggregate resource are estimated are presented in the section on natural aggregate resources in Part II of this report (p. 11), and Appendix D.

Additionally, mining of aggregate resources (production) further reduces the aggregate resource base over time. Production and demographic data from several sources were used in an attempt to quantify historic production trends, and then to project production and consumption trends into the future. The details of these methods are presented in the section on rate of utilization of aggregate resources in Part III of this report (p. 28), and in Appendix E.

Recommended citation: Southwick, D.L., Jouseau, M., Meyer, G.N., Mossler, J.H., and Wahl, T.E., 2000, Aggregate resources inventory of the seven-county metropolitan area, Minnesota: Minnesota Geological Survey Information Circular 46, 91 p.)

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