Uncheking and disabling of Checkbox! Help!!!

[aspengal]

I have a 4 checkboxes. When the 1st one is unchecked, remaining 3 should be disabled and unchecked. Iam not able to achieve both unchecked and disabled at the same time.
This is my code.
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HTML
<td valign="top" nowrap class="bodyTextBold" colspan="5">&nbsp;&nbsp;GW Impact:
<asp:CheckBox ID="chkGWImpact" Runat="server" onclick="ChkGWImpact();setCheckBoxChanged();"></asp:CheckBox>
Dissolved (Delineated):
<asp:CheckBox ID="chkDissolvedDelineated" Runat="server" onclick="setCheckBoxChanged();"></asp:CheckBox>
(Offsite):
<asp:CheckBox ID="chkDissolvedOffsite" Runat="server" onclick="setCheckBoxChanged();"></asp:CheckBox>
(Commingled):
<asp:CheckBox ID="chkDissolvedCommingled" Runat="server" onclick="setCheckBoxChanged();"></asp:CheckBox></td>
<tr valign="top">
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.aspx
function ChkGWImpact(){
document.getElementById('chkDissolvedDelineated').disabled = false;
document.getElementById('chkDissolvedOffsite').disabled = false;
document.getElementById('chkDissolvedCommingled').disabled = false;
if(!document.forms[0].elements['chkGWImpact'].checked){
document.getElementById('chkDissolvedOffsite').disabled = true;
document.getElementById('chkDissolvedOffsite').disabled = true;
document.getElementById('chkDissolvedCommingled').disabled = true;
}
}