Wrap your spans in a div (or other container) and position the container like:
<section id="header">
<div>
<span>HOME</span>
<span class="no-spacing">ABOUT US</span>
<span>PORTFOLIO</span>
<span class="no-spacing">CONTACT US</span>
</div>
</section>
CSS
#header div {
position: absolute;
width:100%;
bottom: 0px;
}