A revealing new report suggests that the foundation operated by the children of Walmart founder Sam Walton is actually being used as a vehicle for the nation’s richest family to skirt billions of dollars in estate taxes.

Walmart 1 Percent, a project of Making Change At Walmart, examined 23 years of tax returns from the Walton Family Foundation, which holds itself out as being interested in improving K-12 education, conservation and improving the quality of life in northwest Arkansas. An analysis of 23 years of foundation tax returns revealed that Sam Walton’s heirs only gave only 0.04 percent of their combined net worth to their own foundation–not even a fraction of what other wealthy Americans give to charity.

Read the full report here or the executive summary here.

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But wait a minute! The Walton Family Foundation has over $2 billion in the bank, and has spent $3.5 billion on the causes it supports. So where does that money come from? According to the report, over 60 percent of the contributions come from 21 trusts set up by Helen Walton, her estate and John Walton’s estate. These trusts, called charitable lead annuity trusts, take advantage of a loophole in the tax code. A portion of the income earned by these trusts, which were worth $8.8 billion in 2012, must go to the Walton Family Foundation for the life of the trusts. When the trusts expire, their beneficiaries–usually the donors’ heirs–receive any underlying assets plus leftover income from the trusts tax-free. By at least one estimate, this will allow Sam’s children and grandchildren to save $3 billion in estate taxes.

To be fair, this tax dodge has been used by several billionaires, which led former Treasury Secretary Larry Summers to cite it as proof that “our estate tax system is broken.” However, given that the Waltons sit on a family fortune worth over $139.9 billion–more than either they or their kids will ever spend–their use of this tactic is particularly obscene.